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

cc/test/layer_tree_pixel_test.cc

Index: cc/test/layer_tree_pixel_test.cc
diff --git a/cc/test/layer_tree_pixel_test.cc b/cc/test/layer_tree_pixel_test.cc
index dd7eedf25e19b18bf9dc1ad205836d861a961bf0..f00ea51461c177665353068ff0add8343a3c7f9b 100644
--- a/cc/test/layer_tree_pixel_test.cc
+++ b/cc/test/layer_tree_pixel_test.cc
@@ -26,7 +26,8 @@ LayerTreePixelTest::LayerTreePixelTest()
 
 LayerTreePixelTest::~LayerTreePixelTest() {}
 
-scoped_ptr<OutputSurface> LayerTreePixelTest::CreateOutputSurface() {
+scoped_ptr<OutputSurface> LayerTreePixelTest::CreateOutputSurface(
+    bool fallback) {
   gfx::Vector2d viewport_offset(20, 10);
   gfx::Size surface_expansion_size(40, 60);
   scoped_ptr<PixelTestOutputSurface> output_surface;