cc: Don't do full tree sync unless needed with impl painting.

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index 36c8d6738bf0de19f6cfe0da05694c025b583a5b..74bbcc4ff3f7f1e1223a65e7a24144b6b756d016 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -980,7 +980,8 @@ void LayerTreeHostImpl::activatePendingTree()
     TRACE_EVENT_ASYNC_END0("cc", "PendingTree", m_pendingTree.get());
 
     m_activeTree->PushPersistedState(m_pendingTree.get());
-    m_activeTree->SetRootLayer(TreeSynchronizer::synchronizeTrees(m_pendingTree->RootLayer(), m_activeTree->DetachLayerTree(), m_activeTree.get()));
+    if (m_needsFullTreeSync)
+        m_activeTree->SetRootLayer(TreeSynchronizer::synchronizeTrees(m_pendingTree->RootLayer(), m_activeTree->DetachLayerTree(), m_activeTree.get()));
     TreeSynchronizer::pushProperties(m_pendingTree->RootLayer(), m_activeTree->RootLayer());
     DCHECK(!m_recycleTree);