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

cc/test/layer_tree_pixel_test.h

Index: cc/test/layer_tree_pixel_test.h
diff --git a/cc/test/layer_tree_pixel_test.h b/cc/test/layer_tree_pixel_test.h
index 72c14d5369ad37cb597daed86d2ed5d726306b7d..b5c177dbd2729f965f0d52b2762061703b5b1f7c 100644
--- a/cc/test/layer_tree_pixel_test.h
+++ b/cc/test/layer_tree_pixel_test.h
@@ -25,9 +25,9 @@ class LayerTreePixelTest : public LayerTreeTest {
   LayerTreePixelTest();
   virtual ~LayerTreePixelTest();
 
-  virtual scoped_ptr<OutputSurface> CreateOutputSurface() OVERRIDE;
+  virtual scoped_ptr<OutputSurface> CreateOutputSurface(bool fallback) OVERRIDE;
   virtual scoped_refptr<cc::ContextProvider>
-      OffscreenContextProviderForMainThread() OVERRIDE;
+  OffscreenContextProviderForMainThread() OVERRIDE;
   virtual scoped_refptr<cc::ContextProvider>
       OffscreenContextProviderForCompositorThread() OVERRIDE;