Add a flag to allow renderer to use software compositor when GL compositor doesn't work.

cc/trees/layer_tree_host_client.h

 // Copyright 2012 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.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_CLIENT_H_
 #define CC_TREES_LAYER_TREE_HOST_CLIENT_H_
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 
 namespace gfx {
 class Vector2d;
 }
 
 namespace cc {
 class ContextProvider;
 class InputHandlerClient;
 class OutputSurface;
 
 class LayerTreeHostClient {
  public:
   virtual void WillBeginFrame() = 0;
   // Marks finishing compositing-related tasks on the main thread. In threaded
   // mode, this corresponds to DidCommit().
   virtual void DidBeginFrame() = 0;
   virtual void Animate(double frame_begin_time) = 0;
   virtual void Layout() = 0;
   virtual void ApplyScrollAndScale(gfx::Vector2d scroll_delta,
                                    float page_scale) = 0;
-  virtual scoped_ptr<OutputSurface> CreateOutputSurface() = 0;
+  // Creates an OutputSurface. If fallback is true, it should attempt to
+  // create an OutputSurface that is guaranteed to initialize correctly.
+  virtual scoped_ptr<OutputSurface> CreateOutputSurface(bool fallback) = 0;
   virtual void DidInitializeOutputSurface(bool success) = 0;
   virtual void WillCommit() = 0;
   virtual void DidCommit() = 0;
   virtual void DidCommitAndDrawFrame() = 0;
   virtual void DidCompleteSwapBuffers() = 0;
 
   // Used only in the single-threaded path.
   virtual void ScheduleComposite() = 0;
 
   // These must always return a valid ContextProvider. But the provider does not
   // need to be capable of creating contexts.
   virtual scoped_refptr<cc::ContextProvider>
       OffscreenContextProviderForMainThread() = 0;
   virtual scoped_refptr<cc::ContextProvider>
       OffscreenContextProviderForCompositorThread() = 0;
 
   // This hook is for testing.
   virtual void DidFailToInitializeOutputSurface() {}
 
  protected:
   virtual ~LayerTreeHostClient() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_CLIENT_H_