Implement getGrContext in SkPaintFilterCanvas

Implement getGrContext to return the context from the original
canvas.

Bug: b/66988832
Test: Ran Android HWUI on a non overlapping View with alpha,
Change-Id: I44ad48e3257004cc5c45c86529b167139f30621a
which is using SkPaintFilterCanvas to draw.
Reviewed-on: https://skia-review.googlesource.com/58841
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Stan Iliev <stani@google.com>
1 file changed