Make image decoders notify bitmaps of pixel changes.

ImageFrame now has a flag to indicate that its pixels were changed, but the SkBitmaps have not yet been notified of the change.
ImageDecoders (JPEG, PNG, GIF, WebP) set this flag when they changed the pixels.
The bitmaps are notified if pixels changed, before a frame is returned from frameBufferAtIndex.

This will make SkPixelRef generationIDs unique for frames
of animated images and for progressive decoded images.

R=hclam@chromium.org,reveman@chromium.org,reed@chromium.org,enne@chromium.org
BUG=307689

Review URL: https://codereview.chromium.org/75303004

git-svn-id: svn://svn.chromium.org/blink/trunk@162410 bbb929c8-8fbe-4397-9dbb-9b2b20218538