some cleanup around GrGpu/GrDrawTarget copySurface

Review URL: https://codereview.chromium.org/749903003
add sample to exercising hittesting

patch from issue 752403002 at patchset 1 (http://crrev.com/752403002#ps1)

BUG=skia:

Review URL: https://codereview.chromium.org/760023002
rename SkDrawable to SkADrawable

BUG=skia:

Review URL: https://codereview.chromium.org/764463002
Update effect key for descriptor to not record offsets.

BUG=skia:

Review URL: https://codereview.chromium.org/760593005
add basic tests for save/restore counting

BUG=skia:

Review URL: https://codereview.chromium.org/763503003