Simplify SkBitmapProvider

Simplify SkBitmapProvider

We're no longer using SkBitmap sources for SkBitmapProvider, so we can
restrict it to SkImage only.

At this point we could also remove it, but I think it'll come in handy
when we add shader subset support.

BUG=skia:5806
R=reed@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4655

Change-Id: I298da253cc8f7c1205b543521e62060a202a9f78
Reviewed-on: https://skia-review.googlesource.com/4655
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>