Revert of third try at landing improved blur rect; this time with more correctness...

Revert of third try at landing improved blur rect; this time with more correctness (https://codereview.chromium.org/331443003/)

Reason for revert:
Failing layout test: https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Linux/32762/layout-test-results/virtual/gpu/fast/canvas/canvas-draw-canvas-on-canvas-shadow-pretty-diff.html

Original issue's description:
> third try at landing improved blur rect; this time with more correctness
>
> BUG=skia:2095
> R=bsalomon@google.com
> TBR=bsalomon
>
> Committed: https://skia.googlesource.com/skia/+/72abfc2b4e7caead660f6b6a05e60d05eaf1a66f

R=bsalomon@google.com, reed@google.com, humper@google.com
TBR=bsalomon@google.com, humper@google.com, reed@google.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:2095

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/333763002