layout: Rewrite whitespace stripping.

This patch makes Servo unconditionally strip whitespace before text run
scanning (assuming that the `white-space` property allows it). Whitespace
stripping during reflow is now only used for handling whitespace at the ends of
lines; reflow now never attempts to handle ignorable whitespace.

Many CSS tests pass now. There are some new failures, however.

The following reference tests now fail due to a pre-existing bug whereby
whitespace is used to calculate the position of inline hypothetical boxes for
elements with `display: inline; position: absolute`:

* `absolute-replaced-height-036.htm`
* `vertical-align-sub-001.htm`
* `vertical-align-super-001.htm`

The following reference tests fail due to a pre-existing bug whereby we don't
handle `font-size: 0` properly in inline reflow:

* `font-size-zero-1.htm`
* `font-size-zero-2.htm`

The following reference test fails due to the fact that it relied on our
incorrect insertion of whitespace to make room for the black background:

* `inline-formatting-context-007.htm`

tests/wpt/metadata-css/css21_dev/html4/absolute-replaced-height-036.htm.ini

@@ -0,0 +1,3 @@
+[absolute-replaced-height-036.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/bidi-breaking-001.htm.ini

@@ -1,3 +0,0 @@
-[bidi-breaking-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/block-in-inline-007.htm.ini

@@ -1,3 +0,0 @@
-[block-in-inline-007.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/c5502-imrgn-r-003.htm.ini

@@ -1,3 +1,4 @@
 [c5502-imrgn-r-003.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "mac": FAIL

tests/wpt/metadata-css/css21_dev/html4/c5525-fltwidth-003.htm.ini

@@ -1,3 +0,0 @@
-[c5525-fltwidth-003.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/content-175.htm.ini

@@ -1,3 +0,0 @@
-[content-175.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/floats-143.htm.ini

@@ -1,3 +0,0 @@
-[floats-143.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/font-weight-applies-to-017.htm.ini

@@ -1,3 +0,0 @@
-[font-weight-applies-to-017.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/inline-formatting-context-007.htm.ini

@@ -1,4 +1,3 @@
 [inline-formatting-context-007.htm]
   type: reftest
-  expected:
-    if os == "mac": FAIL
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/insert-inline-in-blocks-n-inlines-begin-003.htm.ini

@@ -1,3 +0,0 @@
-[insert-inline-in-blocks-n-inlines-begin-003.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/insert-inline-in-blocks-n-inlines-middle-003.htm.ini

@@ -1,3 +0,0 @@
-[insert-inline-in-blocks-n-inlines-middle-003.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/ltr-basic.htm.ini

@@ -1,3 +0,0 @@
-[ltr-basic.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/margin-collapse-137.htm.ini

@@ -1,4 +0,0 @@
-[margin-collapse-137.htm]
-  type: reftest
-  expected:
-    if os == "linux": FAIL

tests/wpt/metadata-css/css21_dev/html4/margin-collapse-138.htm.ini

@@ -1,4 +0,0 @@
-[margin-collapse-138.htm]
-  type: reftest
-  expected:
-    if os == "linux": FAIL

tests/wpt/metadata-css/css21_dev/html4/max-width-percentage-001.htm.ini

@@ -1,3 +0,0 @@
-[max-width-percentage-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/min-width-percentage-001.htm.ini

@@ -1,3 +0,0 @@
-[min-width-percentage-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/rtl-basic.htm.ini

@@ -1,3 +0,0 @@
-[rtl-basic.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/text-transform-capitalize-001.htm.ini

@@ -1,3 +0,0 @@
-[text-transform-capitalize-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/text-transform-lowercase-001.htm.ini

@@ -1,3 +0,0 @@
-[text-transform-lowercase-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/text-transform-uppercase-001.htm.ini

@@ -1,3 +0,0 @@
-[text-transform-uppercase-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/vertical-align-sub-001.htm.ini

@@ -1,3 +1,3 @@
-[abspos-008.htm]
+[vertical-align-sub-001.htm]
   type: reftest
   expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/vertical-align-super-001.htm.ini

@@ -1,3 +1,3 @@
-[abspos-width-005.htm]
+[vertical-align-super-001.htm]
   type: reftest
   expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/white-space-collapsing-bidi-003.htm.ini

@@ -1,3 +0,0 @@
-[white-space-collapsing-bidi-003.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/white-space-normal-003.htm.ini

@@ -1,3 +0,0 @@
-[white-space-normal-003.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/white-space-normal-004.htm.ini

@@ -1,3 +0,0 @@
-[white-space-normal-004.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/white-space-pre-001.htm.ini

@@ -1,3 +0,0 @@
-[white-space-pre-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/white-space-pre-002.htm.ini

@@ -1,3 +0,0 @@
-[white-space-pre-002.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/width-percentage-001.htm.ini

@@ -1,3 +0,0 @@
-[width-percentage-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/width-percentage-002.htm.ini

@@ -1,3 +0,0 @@
-[width-percentage-002.htm]
-  type: reftest
-  expected: FAIL