LTHI's bg animation timer should set the fake thread to impl.

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 53 matching lines @@
         return make_scoped_ptr(new LayerTreeHostImplTimeSourceAdapter(layerTreeHostImpl, timeSource));
     }
     virtual ~LayerTreeHostImplTimeSourceAdapter()
     {
         m_timeSource->setClient(0);
         m_timeSource->setActive(false);
     }
 
     virtual void onTimerTick() OVERRIDE
     {
+        // In single threaded mode we attempt to simulate changing the current
+        // thread by maintaining a fake thread id. When we switch from one
+        // thread to another, we construct DebugScopedSetXXXThread objects that
+        // update the thread id. This lets DCHECKS that ensure we're on the
+        // right thread to work correctly in single threaded mode. The problem
+        // here is that the timer tasks are run via the message loop, and when
+        // they run, we've had no chance to construct a DebugScopedSetXXXThread
+        // object. The result is that we report that we're running on the main
+        // thread. In multi-threaded mode, this timer is run on the compositor
+        // thread, so to keep this consistent in single-threaded mode, we'll
+        // construct a DebugScopedSetImplThread object. There is no need to do
+        // this in multi-threaded mode since the real thread id's will be
+        // correct. In fact, setting fake thread id's interferes with the real
+        // thread id's and causes breakage.
+        scoped_ptr<DebugScopedSetImplThread> setImplThread;
+        if (!m_layerTreeHostImpl->proxy()->hasImplThread())
+            setImplThread.reset(new DebugScopedSetImplThread(m_layerTreeHostImpl->proxy()));
+
         m_layerTreeHostImpl->animate(base::TimeTicks::Now(), base::Time::Now());
     }
 
     void setActive(bool active)
     {
         if (active != m_timeSource->active())
             m_timeSource->setActive(active);
     }
 
 private:
@@ skipping 1547 matching lines @@
 }
 
 skia::RefPtr<SkPicture> LayerTreeHostImpl::capturePicture()
 {
     LayerTreeImpl* tree = pendingTree() ? pendingTree() : activeTree();
     LayerImpl* layer = getNonCompositedContentLayerRecursive(tree->RootLayer());
     return layer ? layer->getPicture() : skia::RefPtr<SkPicture>();
 }
 
 }  // namespace cc