Scale the ALPHA_THRESHOLD filter's region based on the device scale factor.

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index d249a2f472054b60ac0151cc12d55f93c1e7e43f..b75d7ad708e54b46097dcbd08bccd1c5f47fb3b5 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -862,7 +862,7 @@ scoped_ptr<ScopedResource> GLRenderer::GetBackgroundWithFilters(
   }
 
   skia::RefPtr<SkImageFilter> filter = RenderSurfaceFilters::BuildImageFilter(
-      quad->background_filters, device_background_texture->size());
+      quad->background_filters, quad->scale, device_background_texture->size());
 
   SkBitmap filtered_device_background;
   if (apply_background_filters) {
@@ -981,7 +981,7 @@ void GLRenderer::DrawRenderPassQuad(DrawingFrame* frame,
   bool use_color_matrix = false;
   if (!quad->filters.IsEmpty()) {
     skia::RefPtr<SkImageFilter> filter = RenderSurfaceFilters::BuildImageFilter(
-        quad->filters, contents_texture->size());
+        quad->filters, quad->scale, contents_texture->size());
     if (filter) {
       skia::RefPtr<SkColorFilter> cf;