Serialize custom property names as identifiers

Custom property names are defined [1] to be "any valid identifier that
starts with two dashes." As such, they need to be serialized as
identifiers, in order to ensure that reparsing generates a valid custom
property name. Change our serialization of custom property names to use
SerializeIdentifier().

[1]: https://drafts.csswg.org/css-variables/#typedef-custom-property-name

Bug: 828272
Change-Id: Icd830b6b3b38f0414877c6d8dec69efa00318bb3
Reviewed-on: https://chromium-review.googlesource.com/991608Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Chris Nardi <cnardi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547733}

third_party/WebKit/LayoutTests/external/wpt/css/cssom/variable-names-expected.txt

-This is a testharness.js-based test.
-PASS custom property '--a'
-FAIL custom property '--a;b' assert_equals: appears on specified style (after serialization/re-parsing) expected 1 but got 0
-PASS custom property '---'
-FAIL custom property '--\' assert_equals: appears on specified style (after serialization/re-parsing) expected 1 but got 0
-PASS custom property '--ab'
-PASS custom property '--0'
-Harness: the test ran to completion.
-

third_party/WebKit/Source/core/css/StylePropertySerializer.cpp

@@ -26,6 +26,7 @@
 #include <bitset>
 #include "core/css/CSSCustomPropertyDeclaration.h"
 #include "core/css/CSSIdentifierValue.h"
+#include "core/css/CSSMarkup.h"
 #include "core/css/CSSPendingSubstitutionValue.h"
 #include "core/css/CSSValuePool.h"
 #include "core/css/properties/css_property.h"
@@ -179,7 +180,7 @@ String StylePropertySerializer::GetCustomPropertyText(
     result.Append(' ');
   const CSSCustomPropertyDeclaration* value =
       ToCSSCustomPropertyDeclaration(property.Value());
-  result.Append(value->GetName());
+  SerializeIdentifier(value->GetName(), result, is_not_first_decl);
   result.Append(':');
   if (!value->Value())
     result.Append(' ');