[css-typed-om] Make CSSStyleValue.parse work with list-valued properties

Currently, when given a list value, CSSStyleValue.parse returns null,
but the spec says to take the first entry:

https://drafts.css-houdini.org/css-typed-om-1/#dom-cssstylevalue-parse

Bug: 779422
Change-Id: I9f80d1876b9390f1d2684c271d3db3ef4192ddd3
Reviewed-on: https://chromium-review.googlesource.com/743101Reviewed-by: Bugs Nash <bugsnash@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512440}

third_party/WebKit/LayoutTests/typedcssom/stylevalue-objects/parse-parseAll-expected.txt

@@ -13,7 +13,7 @@ FAIL Calling CSSStyleValue.parseAll with an invalid cssText for the given proper
 FAIL Calling CSSStyleValue.parseAll with a valid cssText for the given property returns a valid CSSStyleValue CSSStyleValue[parseMethod] is not a function
 FAIL Calling CSSStyleValue.parseAll with a mixed case cssText returns a valid CSSStyleValue CSSStyleValue[parseMethod] is not a function
 FAIL Calling CSSStyleValue.parseAll with a custom property returns a valid CSSStyleValue CSSStyleValue[parseMethod] is not a function
-FAIL Calling CSSStyleValue.parse with a list-value property returns first list value Cannot read property 'constructor' of null
+PASS Calling CSSStyleValue.parse with a list-value property returns first list value
 FAIL Calling CSSStyleValue.parseAll with a list-value property returns a sequence of values CSSStyleValue.parseAll is not a function
 Harness: the test ran to completion.
 

third_party/WebKit/Source/core/css/cssom/CSSStyleValue.cpp

@@ -46,11 +46,7 @@ ScriptValue CSSStyleValue::parse(ScriptState* script_state,
 
   CSSStyleValueVector style_value_vector =
       StyleValueFactory::CssValueToStyleValueVector(property_id, *css_value);
-  if (style_value_vector.size() != 1) {
-    // TODO(meade): Support returning a CSSStyleValueOrCSSStyleValueSequence
-    // from this function.
-    return ScriptValue::CreateNull(script_state);
-  }
+  DCHECK(!style_value_vector.IsEmpty());
 
   v8::Local<v8::Value> wrapped_value =
       ToV8(style_value_vector[0], script_state->GetContext()->Global(),