Clear WorkerGlobalScope::m_thread in WorkerGlobalScope::dispose()

WorkerGlobalScope::m_thread was intentionally not cleared due to destruction ordering issues in Oilpan (*),
but I think the issue had existed only in the transition period.
This CL adds code to clear WorkerGlobalScope::m_thread in WorkerGlobalScope::dispose().

(*) https://chromium.googlesource.com/chromium/src/+/58f0cf11c7e11bc0dcdd09c08912625b89ba97bb

BUG=

Review-Url: https://codereview.chromium.org/2024033002
Cr-Commit-Position: refs/heads/master@{#398254}

third_party/WebKit/Source/core/workers/WorkerGlobalScope.cpp

@@ -210,16 +210,7 @@ void WorkerGlobalScope::dispose()
     clearScript();
     clearInspector();
     m_eventQueue->close();
-
-    // We do not clear the thread field of the
-    // WorkerGlobalScope. Other objects keep the worker global scope
-    // alive because they need its thread field to check that work is
-    // being carried out on the right thread. We therefore cannot clear
-    // the thread field before all references to the worker global
-    // scope are gone.
-    //
-    // TODO(haraken): It's nasty to keep a raw pointer to WorkerThread
-    // after disposing WorkerGlobalScope. m_thread should be cleared here.
+    m_thread = nullptr;
 }
 
 void WorkerGlobalScope::didEvaluateWorkerScript()