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

cc/layer_tree_host.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.h"
 
 #include "base/command_line.h"
 #include "base/debug/trace_event.h"
 #include "base/message_loop.h"
 #include "base/stl_util.h"
@@ skipping 247 matching lines @@
     // impl tree, so we can't draw yet. Determine this before clearing all evicted backings.
     bool newImplTreeHasNoEvictedResources = !m_contentsTextureManager->linkedEvictedBackingsExist();
 
     m_contentsTextureManager->updateBackingsInDrawingImplTree();
     m_contentsTextureManager->reduceMemory(hostImpl->resourceProvider());
 
     // In impl-side painting, synchronize to the pending tree so that it has
     // time to raster before being displayed.  If no pending tree is needed,
     // synchronization can happen directly to the active tree.
     LayerTreeImpl* syncTree;
-    bool needsFullTreeSync = false;
     if (m_settings.implSidePainting) {
         // Commits should not occur while there is already a pending tree.
         DCHECK(!hostImpl->pendingTree());
         hostImpl->createPendingTree();
         syncTree = hostImpl->pendingTree();
-        // TODO(enne): we could recycle old active trees and keep track for
-        // multiple main thread frames whether a sync is needed
-        needsFullTreeSync = true;
     } else {
         syncTree = hostImpl->activeTree();
-        needsFullTreeSync = m_needsFullTreeSync;
     }
 
-    if (needsFullTreeSync)
+    if (m_needsFullTreeSync)
         syncTree->SetRootLayer(TreeSynchronizer::synchronizeTrees(rootLayer(), syncTree->DetachLayerTree(), syncTree));
     {
         TRACE_EVENT0("cc", "LayerTreeHost::pushProperties");
         TreeSynchronizer::pushProperties(rootLayer(), syncTree->RootLayer());
     }
 
+    syncTree->set_needs_full_tree_sync(m_needsFullTreeSync);
     m_needsFullTreeSync = false;
 
     if (m_rootLayer && m_hudLayer)
         syncTree->set_hud_layer(static_cast<HeadsUpDisplayLayerImpl*>(LayerTreeHostCommon::findLayerInSubtree(syncTree->RootLayer(), m_hudLayer->id())));
     else
         syncTree->set_hud_layer(0);
 
     syncTree->set_source_frame_number(commitNumber());
     syncTree->set_background_color(m_backgroundColor);
     syncTree->set_has_transparent_background(m_hasTransparentBackground);
@@ skipping 567 matching lines @@
     for (size_t childIndex = 0; childIndex < layer->children().size(); ++childIndex)
         setAnimationEventsRecursive(events, layer->children()[childIndex].get(), wallClockTime);
 }
 
 skia::RefPtr<SkPicture> LayerTreeHost::capturePicture()
 {
     return m_proxy->capturePicture();
 }
 
 }  // namespace cc