Serialize font-variation-settings with double-quotes

According to the CSSOM spec [1], all strings should be serialized with
double-quotes. The axis name in font-variation-settings was previously
serialized with single-quotes; change this to double-quotes to match
the spec and non-WebKit browsers.

[1]: https://drafts.csswg.org/cssom/#common-serializing-idioms

Bug: 806412
Change-Id: Ib0da551c2799a9dc4e273ac150972a7bd5c7a34f
Reviewed-on: https://chromium-review.googlesource.com/900843Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Chris Nardi <cnardi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534803}

third_party/WebKit/LayoutTests/fast/css/resources/font-variation-settings-css-support.js

 var validWriteExpectations = [ "normal",
-                               "'abcd' 1",
-                               "'abcd' 0, 'efgh' 1",
-                               "'abcd' 0.3, 'efgh' 1.4",
-                               "'abcd' -1000, 'efgh' -10000",
-                               "'abcd' 0.3, 'abcd' 0.4, 'abcd' 0.5, 'abcd' 0.6, 'abcd' 0.7" ];
+                               '"abcd" 1',
+                               '"abcd" 0, "efgh" 1',
+                               '"abcd" 0.3, "efgh" 1.4',
+                               '"abcd" -1000, "efgh" -10000',
+                               '"abcd" 0.3, "abcd" 0.4, "abcd" 0.5, "abcd" 0.6, "abcd" 0.7' ];
 
 var writeInvalidExpectations = ["none",
                                 "'abc' 1",

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

@@ -4,6 +4,7 @@
 
 #include "core/css/CSSFontVariationValue.h"
 
+#include "core/css/CSSMarkup.h"
 #include "platform/wtf/text/StringBuilder.h"
 
 namespace blink {
@@ -15,9 +16,8 @@ CSSFontVariationValue::CSSFontVariationValue(const AtomicString& tag,
 
 String CSSFontVariationValue::CustomCSSText() const {
   StringBuilder builder;
-  builder.Append('\'');
-  builder.Append(tag_);
-  builder.Append("' ");
+  SerializeString(tag_, builder);
+  builder.Append(' ');
   builder.AppendNumber(value_);
   return builder.ToString();
 }