Update app_units to 0.3.1

The height test is removed in mime_sniffing_font_context because of rounding
changes in app_units. The rest of that test is still useful though, given it
will timeout if the font fails to be sniffed.

tests/wpt/metadata-css/css21_dev/html4/background-repeat-repeat-x.htm.ini

@@ -1,4 +1,3 @@
 [background-repeat-repeat-x.htm]
   type: reftest
-  expected:
-    if os == "mac": FAIL
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/background-repeat-repeat-y.htm.ini

@@ -1,4 +1,3 @@
 [background-repeat-repeat-y.htm]
   type: reftest
-  expected:
-    if os == "mac": FAIL
+  expected: FAIL

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

@@ -0,0 +1,4 @@
+[c5525-fltwidth-003.htm]
+  type: reftest
+  expected:
+    if os == "linux": FAIL

tests/wpt/metadata-css/css21_dev/html4/floats-028.htm.ini

@@ -1,3 +1,4 @@
 [floats-028.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "mac": FAIL