Implement GPU occluded blur mask filter

Spawned off: https://codereview.chromium.org/2214163003/ (Minor clean up related to blur mask filters)

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2201133002

Review-Url: https://codereview.chromium.org/2201133002
diff --git a/gm/occludedrrectblur.cpp b/gm/occludedrrectblur.cpp
index 937b179..e21a014 100644
--- a/gm/occludedrrectblur.cpp
+++ b/gm/occludedrrectblur.cpp
@@ -46,7 +46,7 @@
 
     // draw the blur
     SkPaint paint;
-    paint.setMaskFilter(SkBlurMaskFilter::Make(kNormal_SkBlurStyle, kBlurSigma)); //, occRect));
+    paint.setMaskFilter(SkBlurMaskFilter::Make(kNormal_SkBlurStyle, kBlurSigma, occRect));
     canvas->drawRRect(rr, paint);
 
     // draw the stroked geometry of the full occluder