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

cc/trees/layer_tree_host_client.h

Index: cc/trees/layer_tree_host_client.h
diff --git a/cc/trees/layer_tree_host_client.h b/cc/trees/layer_tree_host_client.h
index 680552e4efad6d0e9ac595c939f20f7351989ee3..9e11f7cbd5d219890a37f48c9a014c6783a518df 100644
--- a/cc/trees/layer_tree_host_client.h
+++ b/cc/trees/layer_tree_host_client.h
@@ -27,7 +27,9 @@ class LayerTreeHostClient {
   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;