Auto merge of #11428 - g-k:csstext, r=SimonSapin

csstext

- [x] These changes fix #4431.
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy --faster` reports one error for the `css_properties_accessors!` macro not having a spec link

```
$ git log -1 --format=oneline
91fb9bf1d3 fixup! implement cssText
$ ./mach build -d
   Compiling style v0.0.1 (file:///Users/greg/servo/components/style)
   Compiling gfx v0.0.1 (file:///Users/greg/servo/components/gfx)
   Compiling script v0.0.1 (file:///Users/greg/servo/components/script)
   Compiling layout_traits v0.0.1 (file:///Users/greg/servo/components/layout_traits)
   Compiling compositing v0.0.1 (file:///Users/greg/servo/components/compositing)
   Compiling glutin_app v0.0.1 (file:///Users/greg/servo/ports/glutin)
   Compiling constellation v0.0.1 (file:///Users/greg/servo/components/constellation)
   Compiling layout v0.0.1 (file:///Users/greg/servo/components/layout)
   Compiling servo v0.0.1 (file:///Users/greg/servo/components/servo)
Build completed in 0:05:11.475584
$ ./mach test-tidy --faster
Checking files for tidiness...
./components/script/dom/cssstyledeclaration.rs:386: method declared in webidl is missing a comment with a specification link
  Progress: 100% (12/12)
```

- [x] There are tests for these changes.  More CSSOM tests pass, but others fail that probably shouldn't:

* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/index-002.htm` and a bunch of the other tests in `/css-tests/cssom-1_dev/html/ crash when run individually
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/cssom-cssText-serialize.htm` fails to strip a trailing semicolon (`left: 10px` vs `left: 10px;`)
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/index-001.htm` shared shorthand values aren't coalesced (`margin: 20px` vs. `margin: 20px 20px 20px 20px`)
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/cssstyledeclaration-csstext.htm` also crashes and fails for `color: red;` vs. `color: RED;`,  preserving declaration insertion order, whitespace in the value, and setting an unknown style property.

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tests/wpt/metadata-css/cssom-1_dev/html/computed-style-001.htm.ini

@@ -1,5 +0,0 @@
-[computed-style-001.htm]
-  type: testharness
-  [read_only]
-    expected: FAIL
-

tests/wpt/metadata-css/cssom-1_dev/html/cssstyledeclaration-csstext.htm.ini

@@ -1,17 +1,8 @@
 [cssstyledeclaration-csstext.htm]
   type: testharness
-  [uppercase property]
-    expected: FAIL
-
   [uppercase value]
     expected: FAIL
 
-  [overwriting with invalid value]
-    expected: FAIL
-
-  [use rgb]
-    expected: FAIL
-
   [cssText order]
     expected: FAIL
 

tests/wpt/metadata-css/cssom-1_dev/html/cssstyledeclaration-mutability.htm.ini

@@ -1,8 +1,5 @@
 [cssstyledeclaration-mutability.htm]
   type: testharness
-  [HTMLElement's CSSStyleDeclaration is mutable]
-    expected: FAIL
-
   [StyleSheet's CSSStyleDeclaration is mutable]
     expected: FAIL
 

tests/wpt/metadata-css/cssom-1_dev/html/index-002.htm.ini

@@ -30,27 +30,15 @@
   [border is expected to be border-width: 1px;]
     expected: FAIL
 
-  [overflow is expected to be overflow: scroll hidden;]
-    expected: FAIL
-
   [overflow is expected to be overflow: scroll;]
     expected: FAIL
 
   [outline is expected to be outline: blue dotted 2px;]
     expected: FAIL
 
-  [margin is expected to be margin: 1px 2px 3px 4px;]
-    expected: FAIL
-
   [list is expected to be list-style: circle inside;]
     expected: FAIL
 
-  [list is expected to be list-style-type: lower-alpha;]
-    expected: FAIL
-
   [font-family is expected to be font-family: sans-serif; line-height: 2em; font-size: 3em; font-style: italic; font-weight: bold;]
     expected: FAIL
 
-  [padding is expected to be padding: 1px 2px 3px 4px;]
-    expected: FAIL
-

tests/wpt/metadata-css/cssom-1_dev/html/interfaces.htm.ini

@@ -474,9 +474,6 @@
   [CSSNamespaceRule interface: attribute prefix]
     expected: FAIL
 
-  [CSSStyleDeclaration interface: attribute cssText]
-    expected: FAIL
-
   [CSSStyleDeclaration interface: attribute parentRule]
     expected: FAIL