add test and fix for canvas::isClipEmpty

BUG=skia:

Change-Id: I4e57e0da7111e861fbae1b88574bc1803442243c
Reviewed-on: https://skia-review.googlesource.com/10046
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Derek Sollenberger <djsollen@google.com>
3 files changed