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

cc/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layer_tree_host_impl.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
@@ skipping 962 matching lines @@
 
     activatePendingTree();
 }
 
 void LayerTreeHostImpl::activatePendingTree()
 {
     CHECK(m_pendingTree);
     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);
 
     m_pendingTree->pushPropertiesTo(m_activeTree.get());
 
     // Now that we've synced everything from the pending tree to the active
     // tree, rename the pending tree the recycle tree so we can reuse it on the
     // next sync.
     m_pendingTree.swap(m_recycleTree);
     m_recycleTree->ClearRenderSurfaces();
@@ skipping 687 matching lines @@
     LayerImpl* layer = getNonCompositedContentLayerRecursive(tree->RootLayer());
     return layer ? layer->getPicture() : skia::RefPtr<SkPicture>();
 }
 
 void LayerTreeHostImpl::savePaintTime(const base::TimeDelta& totalPaintTime)
 {
     m_paintTimeCounter->SavePaintTime(totalPaintTime);
 }
 
 }  // namespace cc