Auto merge of #6798 - dzbarsky:gCS, r=pcwalton

Implement getComputedStyle

It's not quite done but can probably be reviewed anyway.
I still need to finish up a few of the ToCss impls, I just got lazy and wanted to make sure things worked.

The computation of the used values is definitely not right, I'm going to investigate that.

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tests/wpt/metadata-css/css21_dev/html4/events-006.htm.ini

@@ -1,9 +1,8 @@
 [events-006.htm]
   type: testharness
-  expected: TIMEOUT
   [transition padding-left on :before / events]
-    expected: NOTRUN
+    expected: FAIL
 
   [transition padding-left on :after / events]
-    expected: NOTRUN
+    expected: FAIL
 

tests/wpt/metadata-css/css21_dev/html4/pseudo-elements-001.htm.ini

@@ -1,15 +1,14 @@
 [pseudo-elements-001.htm]
   type: testharness
-  expected: TIMEOUT
   [transition padding-left on :before / values]
-    expected: NOTRUN
+    expected: FAIL
 
   [transition padding-left on :after / values]
-    expected: NOTRUN
+    expected: FAIL
 
   [transition padding-left on :before, changing content / values]
-    expected: NOTRUN
+    expected: FAIL
 
   [transition padding-left on :after, changing content / values]
-    expected: NOTRUN
+    expected: FAIL