Use a lock to deal with concurrent access to the m_evictedBackings

cc/thread_proxy.cc

Index: cc/thread_proxy.cc
diff --git a/cc/thread_proxy.cc b/cc/thread_proxy.cc
index 2281164263a28f51999db0dc8d1923bc5618a32b..1056fa8b49d64fa6b504c3ee95f60a412d7185eb 100644
--- a/cc/thread_proxy.cc
+++ b/cc/thread_proxy.cc
@@ -516,9 +516,6 @@ void ThreadProxy::scheduledActionBeginFrame()
     beginFrameState->implTransform = m_layerTreeHostImpl->implTransform();
     DCHECK_GT(m_layerTreeHostImpl->memoryAllocationLimitBytes(), 0u);
     beginFrameState->memoryAllocationLimitBytes = m_layerTreeHostImpl->memoryAllocationLimitBytes();
-    if (m_layerTreeHost->contentsTextureManager())
-        m_layerTreeHost->contentsTextureManager()->getEvictedBackings(beginFrameState->evictedContentsTexturesBackings);
-
     m_mainThreadProxy->postTask(FROM_HERE, base::Bind(&ThreadProxy::beginFrame, base::Unretained(this), base::Passed(beginFrameState.Pass())));
 
     if (m_beginFrameCompletionEventOnImplThread) {
@@ -534,13 +531,6 @@ void ThreadProxy::beginFrame(scoped_ptr<BeginFrameAndCommitState> beginFrameStat
     if (!m_layerTreeHost)
         return;
 
-    // Be sure to never early-out of unlinking evicted textures whenever, so that
-    // the each evicted backing is sent to the main thread exactly once.
-    if (beginFrameState && !beginFrameState->evictedContentsTexturesBackings.empty()) {
-        DCHECK(m_layerTreeHost->contentsTextureManager());
-        m_layerTreeHost->contentsTextureManager()->unlinkEvictedBackings(beginFrameState->evictedContentsTexturesBackings);
-    }
-
     if (m_deferCommits) {
         m_pendingDeferredCommit = beginFrameState.Pass();
         m_layerTreeHost->didDeferCommit();
@@ -595,6 +585,10 @@ void ThreadProxy::beginFrame(scoped_ptr<BeginFrameAndCommitState> beginFrameStat
         return;
     }
 
+    // Unlink any backings that the impl thread has evicted, so that we know to re-paint
+    // them in updateLayers.
+    m_layerTreeHost->contentsTextureManager()->unlinkAndClearEvictedBackings();
+
     scoped_ptr<ResourceUpdateQueue> queue = make_scoped_ptr(new ResourceUpdateQueue);
     m_layerTreeHost->updateLayers(*(queue.get()), beginFrameState ? beginFrameState->memoryAllocationLimitBytes : 0);