cc cleanup: simplify mask_uv_scale and MaskUVRect().

cc/quads/render_pass_draw_quad.cc

Index: cc/quads/render_pass_draw_quad.cc
diff --git a/cc/quads/render_pass_draw_quad.cc b/cc/quads/render_pass_draw_quad.cc
index a0070d6991f059e4bc69108c0e43399ea2bda34e..a34669140483c740314bd5783dfe01015f6e4879 100644
--- a/cc/quads/render_pass_draw_quad.cc
+++ b/cc/quads/render_pass_draw_quad.cc
@@ -73,11 +73,7 @@ void RenderPassDrawQuad::SetAll(const SharedQuadState* shared_quad_state,
 }
 
 gfx::RectF RenderPassDrawQuad::MaskUVRect() const {
-  gfx::RectF mask_uv_rect((mask_uv_scale.x() * rect.x()) / rect.width(),
-                          (mask_uv_scale.y() * rect.y()) / rect.height(),
-                          mask_uv_scale.x(),
-                          mask_uv_scale.y());
-  return mask_uv_rect;
+  return gfx::ScaleRect(gfx::RectF(rect), mask_uv_scale.x(), mask_uv_scale.y());
 }
 
 const RenderPassDrawQuad* RenderPassDrawQuad::MaterialCast(