change clip-bounds getters to always return the rect

change clip-bounds getters to always return the rect
(actually fixes undefined result in getClipBounds)

future CLs
- update all callers to new apis
- move/rename virtuals

BUG=skia:

DOCS_PREVIEW= https://skia.org/?cl=7400

Change-Id: I45b93014e915c0d1c36d97d948c9ac8931f23258
Reviewed-on: https://skia-review.googlesource.com/7400
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>