Unset CompositorWorkerGlobalScope during WorkerGlobalScope::dipose.

third_party/WebKit/Source/web/CompositorProxyClientImpl.cpp

Index: third_party/WebKit/Source/web/CompositorProxyClientImpl.cpp
diff --git a/third_party/WebKit/Source/web/CompositorProxyClientImpl.cpp b/third_party/WebKit/Source/web/CompositorProxyClientImpl.cpp
index 4a621c5f1ab5d7c8d7e5e260394e8bf827287093..e0544e25a612c8d10a201d79aec0aa74b84e88bf 100644
--- a/third_party/WebKit/Source/web/CompositorProxyClientImpl.cpp
+++ b/third_party/WebKit/Source/web/CompositorProxyClientImpl.cpp
@@ -53,10 +53,10 @@ DEFINE_TRACE(CompositorProxyClientImpl)
 void CompositorProxyClientImpl::setGlobalScope(WorkerGlobalScope* scope)
 {
     TRACE_EVENT0("compositor-worker", "CompositorProxyClientImpl::setGlobalScope");
-    DCHECK(!m_globalScope);
-    DCHECK(scope);
+    DCHECK(!m_globalScope || !scope);
     m_globalScope = static_cast<CompositorWorkerGlobalScope*>(scope);
-    m_mutator->registerProxyClient(this);
+    if (m_globalScope)
+        m_mutator->registerProxyClient(this);
 }
 
 void CompositorProxyClientImpl::requestAnimationFrame()