Add flag for drawing layers to screen with Ganesh

cc/layers/picture_layer_impl_unittest.cc

Index: cc/layers/picture_layer_impl_unittest.cc
diff --git a/cc/layers/picture_layer_impl_unittest.cc b/cc/layers/picture_layer_impl_unittest.cc
index 1fd20f1faead7641adcfcd7e80ba1d4d01b1b687..3631f9d1d021ed110b7c7f5a9533f8fd69e988eb 100644
--- a/cc/layers/picture_layer_impl_unittest.cc
+++ b/cc/layers/picture_layer_impl_unittest.cc
@@ -224,10 +224,8 @@ class PictureLayerImplTest : public testing::Test {
       // at content scale 1.0, the playback canvas never receives content from
       // neighboring tiles which indicates that the tile grid embedded in
       // SkPicture is perfectly aligned with the compositor's tiles.
-      // Note: There are two rects: the initial clear and the explicitly
-      // recorded rect. We only care about the second one.
-      EXPECT_EQ(2u, mock_canvas.rects_.size());
-      EXPECT_RECT_EQ(*rect_iter, mock_canvas.rects_[1]);
+      EXPECT_EQ(1u, mock_canvas.rects_.size());
+      EXPECT_RECT_EQ(*rect_iter, mock_canvas.rects_[0]);
       rect_iter++;
     }
   }