Fix setting css-wide keywords on registered custom properties

This patch fixes setting of css-wide keywords on registered custom
properties, which was broken by crrev.com/2607403002.

BUG=679627

Review-Url: https://codereview.chromium.org/2624213004
Cr-Commit-Position: refs/heads/master@{#443204}

third_party/WebKit/LayoutTests/custom-properties/registered-property-cssom.html

@@ -67,6 +67,8 @@ test(function() {
   assert_equals(computedStyle.getPropertyValue('--length'), '10px');
   sheetStyle.setProperty('--length', '20px');
   assert_equals(computedStyle.getPropertyValue('--length'), '20px');
+  sheetStyle.setProperty('--length', 'initial');
+  assert_equals(computedStyle.getPropertyValue('--length'), '0px');
 }, "Stylesheets can be modified by CSSOM");
 
 test(function() {
@@ -76,5 +78,8 @@ test(function() {
   assert_equals(inlineStyle.getPropertyValue('--color'), 'pink');
   assert_equals(computedStyle.getPropertyValue('--length'), '30px');
   assert_equals(computedStyle.getPropertyValue('--color'), 'pink');
+  inlineStyle.setProperty('--color', 'inherit');
+  assert_equals(inlineStyle.getPropertyValue('--color'), 'inherit');
+  assert_equals(computedStyle.getPropertyValue('--color'), 'blue');
 }, "Valid values can be set on inline styles");
 </script>

third_party/WebKit/Source/core/css/parser/CSSParserImpl.cpp

@@ -81,7 +81,9 @@ MutableStylePropertySet::SetResult CSSParserImpl::parseVariableValue(
   bool didParse = false;
   bool didChange = false;
   if (!parser.m_parsedProperties.isEmpty()) {
-    if (registry) {
+    const CSSCustomPropertyDeclaration* parsedDeclaration =
+        toCSSCustomPropertyDeclaration(parser.m_parsedProperties[0].value());
+    if (parsedDeclaration->value() && registry) {
       const PropertyRegistry::Registration* registration =
           registry->registration(propertyName);
       // TODO(timloh): This is a bit wasteful, we parse the registered property