Revert of SkCanvas::clear() should call down to devices even when the clip is empty. (patchset #1 id:1 of https://codereview.chromium.org/735233003/)

Reason for revert:
Chrome unit_tests failures on Mac & Win, blocking the roll: http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_ng/builds/5613

Original issue's description:
> SkCanvas::clear() should call down to devices even when the clip is empty.
>
> SkDrawIter had a lovely, completely unused, option for this.  Oversight?
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/378e70d8e561b72a904286ea231946f5c8345b9a

TBR=reed@google.com,mtklein@google.com,mtklein@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/748733002