Use a lock to deal with concurrent access to the m_evictedBackings

cc/single_thread_proxy.cc

Index: cc/single_thread_proxy.cc
diff --git a/cc/single_thread_proxy.cc b/cc/single_thread_proxy.cc
index 7e53375f2527ce91ffa7d7922c285c8fd4d04957..d0bea46a299c90c78f8213817990544052029681 100644
--- a/cc/single_thread_proxy.cc
+++ b/cc/single_thread_proxy.cc
@@ -342,13 +342,7 @@ bool SingleThreadProxy::commitAndComposite()
     if (!m_layerTreeHost->initializeRendererIfNeeded())
         return false;
 
-    // Unlink any texture backings that were deleted
-    PrioritizedResourceManager::BackingList evictedContentsTexturesBackings;
-    {
-        DebugScopedSetImplThread impl(this);
-        m_layerTreeHost->contentsTextureManager()->getEvictedBackings(evictedContentsTexturesBackings);
-    }
-    m_layerTreeHost->contentsTextureManager()->unlinkEvictedBackings(evictedContentsTexturesBackings);
+    m_layerTreeHost->contentsTextureManager()->unlinkAndClearEvictedBackings();
 
     scoped_ptr<ResourceUpdateQueue> queue = make_scoped_ptr(new ResourceUpdateQueue);
     m_layerTreeHost->updateLayers(*(queue.get()), m_layerTreeHostImpl->memoryAllocationLimitBytes());