Mac: Don't repaint scrollbars every frame

cc/output/ca_layer_overlay.cc

Index: cc/output/ca_layer_overlay.cc
diff --git a/cc/output/ca_layer_overlay.cc b/cc/output/ca_layer_overlay.cc
index f51c8ff86f2756ddcb39a20a20ded0d98addfc2d..584a082c10a9efb5d337eb854fc0e27e6a9f12ca 100644
--- a/cc/output/ca_layer_overlay.cc
+++ b/cc/output/ca_layer_overlay.cc
@@ -66,6 +66,11 @@ bool FromTextureQuad(ResourceProvider* resource_provider,
   ca_layer_overlay->contents_rect =
       BoundingRect(quad->uv_top_left, quad->uv_bottom_right);
   ca_layer_overlay->background_color = quad->background_color;
+  for (int i = 1; i < 4; ++i) {
+    if (quad->vertex_opacity[i] != quad->vertex_opacity[0])
+      return false;
+  }
+  ca_layer_overlay->opacity *= quad->vertex_opacity[0];
   return true;
 }