cc cleanup: simplify mask_uv_scale and MaskUVRect().

cc/layers/render_surface_impl.cc

Index: cc/layers/render_surface_impl.cc
diff --git a/cc/layers/render_surface_impl.cc b/cc/layers/render_surface_impl.cc
index 8e6db1eed24d757d6f175512d1f9ff5ee7063a1f..8d8a95bbbc48511966b6ec1373c722d403e12167 100644
--- a/cc/layers/render_surface_impl.cc
+++ b/cc/layers/render_surface_impl.cc
@@ -237,9 +237,8 @@ void RenderSurfaceImpl::AppendQuads(RenderPass* render_pass,
     gfx::SizeF unclipped_mask_target_size = gfx::ScaleSize(
         gfx::SizeF(owning_layer_->bounds()), owning_layer_draw_scale.x(),
         owning_layer_draw_scale.y());
-    mask_uv_scale = gfx::Vector2dF(
-        content_rect_.width() / unclipped_mask_target_size.width(),
-        content_rect_.height() / unclipped_mask_target_size.height());
+    mask_uv_scale = gfx::Vector2dF(1.0f / unclipped_mask_target_size.width(),
+                                   1.0f / unclipped_mask_target_size.height());
   }
 
   DCHECK(owning_layer_draw_transform.IsScale2d());