[css-typed-om] Throw SyntaxError on parsing invalid cssText.

Currently we return null when calling CSSStyleValue.parse with an
invalid cssText. The spec says to throw a SyntaxError:

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

Bug: 775806
Change-Id: Iab99af48ce162fa1810ac678e63475e1660d9d95
Reviewed-on: https://chromium-review.googlesource.com/743082Reviewed-by: Bugs Nash <bugsnash@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512438}

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

@@ -2,7 +2,7 @@ This is a testharness.js-based test.
 PASS Calling CSSStyleValue.parse with an empty string throws a TypeError
 PASS Calling CSSStyleValue.parse with an unsupported CSS property throws a TypeError
 PASS Calling CSSStyleValue.parse with a CSS shorthand throws a TypeError
-FAIL Calling CSSStyleValue.parse with an invalid cssText for the given property throws a SyntaxError assert_throws: function "() => CSSStyleValue[parseMethod]('width', '10deg')" did not throw
+PASS Calling CSSStyleValue.parse with an invalid cssText for the given property throws a SyntaxError
 PASS Calling CSSStyleValue.parse with a valid cssText for the given property returns a valid CSSStyleValue
 PASS Calling CSSStyleValue.parse with a mixed case cssText returns a valid CSSStyleValue
 FAIL Calling CSSStyleValue.parse with a custom property returns a valid CSSStyleValue Failed to execute 'parse' on 'CSSStyleValue': Invalid property name

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

@@ -36,8 +36,13 @@ ScriptValue CSSStyleValue::parse(ScriptState* script_state,
 
   const CSSValue* css_value =
       CSSParser::ParseSingleValue(property_id, value, StrictCSSParserContext());
-  if (!css_value)
+  if (!css_value) {
+    exception_state.ThrowDOMException(
+        kSyntaxError, "The value provided ('" + value +
+                          "') could not be parsed as a '" + property_name +
+                          "'.");
     return ScriptValue::CreateNull(script_state);
+  }
 
   CSSStyleValueVector style_value_vector =
       StyleValueFactory::CssValueToStyleValueVector(property_id, *css_value);