GMs: Call SkBitmap::notifyPixelsChanged()

Motivation:  SkPDF uses the generation ID to to  de-duplicate bitmaps.
gammaencodedpremul.cpp was not drawing correctly before this change.

Change-Id: I6c59758ee0693f18e6ed7c51b22ee748af604ddc
Reviewed-on: https://skia-review.googlesource.com/22207
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
1 file changed