Fix crash in AsyncCallTracker::didEnqueueMutationRecord

The method should be allowed to execute JS since it needs it to collect call stack.

BUG=525305

Review URL: https://codereview.chromium.org/1313693007

git-svn-id: svn://svn.chromium.org/blink/trunk@201280 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/inspector/AsyncCallTracker.cpp

@@ -310,6 +310,9 @@ void AsyncCallTracker::didEnqueueMutationRecord(ExecutionContext* context, Mutat
     ExecutionContextData* data = createContextDataIfNeeded(context);
     if (data->m_mutationObserverCallChains.contains(observer))
         return;
+    Optional<ScriptForbiddenScope::AllowUserAgentScript> allowScripting;
+    if (isMainThread())
+        allowScripting.emplace();
     int operationId = m_debuggerAgent->traceAsyncOperationStarting(enqueueMutationRecordName);
     data->m_mutationObserverCallChains.set(observer, operationId);
 }