Enable Oilpan per thread heap on CompositorWorker

Design doc: https://docs.google.com/document/d/14MzxMuUV9e5ZgxbZL3-jCMKa3S-UJGsbYSmRodLh9uc/edit#
PSA email: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/wROZOp-YKSc

BUG=591606

Review-Url: https://codereview.chromium.org/2344443003
Cr-Commit-Position: refs/heads/master@{#419098}

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

@@ -55,7 +55,7 @@ WorkerBackingThread::WorkerBackingThread(const char* name, bool shouldCallGCOnSh
 }
 
 WorkerBackingThread::WorkerBackingThread(WebThread* thread, bool shouldCallGCOnShutdown)
-    : m_backingThread(WebThreadSupportingGC::createForThread(thread))
+    : m_backingThread(WebThreadSupportingGC::createForThread(thread, true))
     , m_isOwningThread(false)
     , m_shouldCallGCOnShutdown(shouldCallGCOnShutdown)
 {