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

cc/test/layer_tree_test.cc

Index: cc/test/layer_tree_test.cc
diff --git a/cc/test/layer_tree_test.cc b/cc/test/layer_tree_test.cc
index ed97752eb2a5dfd5b3b829a3171df69d0a14a74f..70452a0987a693095b905575cb5f595d04111e99 100644
--- a/cc/test/layer_tree_test.cc
+++ b/cc/test/layer_tree_test.cc
@@ -251,7 +251,8 @@ class LayerTreeHostClientForTesting : public LayerTreeHostClient {
     test_hooks_->ApplyScrollAndScale(scroll_delta, scale);
   }
 
-  virtual scoped_ptr<OutputSurface> CreateOutputSurface() OVERRIDE {
+  virtual scoped_ptr<OutputSurface> CreateOutputSurface(bool fallback)
+      OVERRIDE {
     return test_hooks_->CreateOutputSurface();
   }