[cc] Move root layer pointer to LayerTreeImpl

This is the first of a series of patches to move tree-specific state from
LayerTreeHostImpl to LayerTreeImpl. The LayerTreeHostImpl has always been
imagined to be a "View" aka LayerTreeView(Impl) and will keep ahold of stuff
that is view-specific.  The LayerTreeImpl, in constrast, will store state that
is specific to a specific tree version.

BUG=155209
R=enne


Review URL: https://chromiumcodereview.appspot.com/11450015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@171593 0039d316-1c4b-4281-b951-d872f2087c98

cc/cc.gyp

@@ -102,6 +102,8 @@
       'layer_tree_host_common.h',
       'layer_tree_host_impl.cc',
       'layer_tree_host_impl.h',
+      'layer_tree_impl.cc',
+      'layer_tree_impl.h',
       'layer_updater.cc',
       'layer_updater.h',
       'managed_memory_policy.cc',

cc/layer_tree_host.cc

@@ -383,16 +383,16 @@ void LayerTreeHost::finishCommitOnImplThread(LayerTreeHostImpl* hostImpl)
     m_needsFullTreeSync = false;
 
     if (m_rootLayer && m_hudLayer)
-        hostImpl->setHudLayer(static_cast<HeadsUpDisplayLayerImpl*>(LayerTreeHostCommon::findLayerInSubtree(hostImpl->rootLayer(), m_hudLayer->id())));
+        hostImpl->activeTree()->set_hud_layer(static_cast<HeadsUpDisplayLayerImpl*>(LayerTreeHostCommon::findLayerInSubtree(hostImpl->rootLayer(), m_hudLayer->id())));
     else
-        hostImpl->setHudLayer(0);
+        hostImpl->activeTree()->set_hud_layer(0);
 
     // We may have added an animation during the tree sync. This will cause both layer tree hosts
     // to visit their controllers.
     if (rootLayer() && m_needsAnimateLayers)
         hostImpl->setNeedsAnimateLayers();
 
-    hostImpl->setSourceFrameNumber(commitNumber());
+    hostImpl->activeTree()->set_source_frame_number(commitNumber());
     hostImpl->setViewportSize(layoutViewportSize(), deviceViewportSize());
     hostImpl->setDeviceScaleFactor(deviceScaleFactor());
     hostImpl->setPageScaleFactorAndLimits(m_pageScaleFactor, m_minPageScaleFactor, m_maxPageScaleFactor);