Make CSSParserContext be garbage collected.

third_party/WebKit/Source/core/dom/CSSSelectorWatch.cpp

Index: third_party/WebKit/Source/core/dom/CSSSelectorWatch.cpp
diff --git a/third_party/WebKit/Source/core/dom/CSSSelectorWatch.cpp b/third_party/WebKit/Source/core/dom/CSSSelectorWatch.cpp
index 2570be9cb0e27ee525d195507fce21826869b37d..bd1ff5f126b00b792c9c44bc5a8c6895b360893d 100644
--- a/third_party/WebKit/Source/core/dom/CSSSelectorWatch.cpp
+++ b/third_party/WebKit/Source/core/dom/CSSSelectorWatch.cpp
@@ -151,9 +151,10 @@ void CSSSelectorWatch::watchCSSSelectors(const Vector<String>& selectors) {
   StylePropertySet* callbackPropertySet =
       ImmutableStylePropertySet::create(nullptr, 0, UASheetMode);
 
+  CSSParserContext* context = CSSParserContext::create(UASheetMode);
   for (const auto& selector : selectors) {
-    CSSSelectorList selectorList = CSSParser::parseSelector(
-        CSSParserContext(UASheetMode, nullptr), nullptr, selector);
+    CSSSelectorList selectorList =
+        CSSParser::parseSelector(context, nullptr, selector);
     if (!selectorList.isValid())
       continue;