Make generation IDs of animated GIF frames unique.

In some cases GIFImageDecoder::initFrameBuffer/WEBImageDecoder::initFrameBuffer copy the content of the
previous frame as the starting state for the current frame. The generation ID
of the two bitmaps is identical after that copy. We need to call
SkBitmap::notifyPixelsChanged to make sure that the new frame gets a unique
generation ID.

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

Review URL: https://codereview.chromium.org/65713004

git-svn-id: svn://svn.chromium.org/blink/trunk@162065 bbb929c8-8fbe-4397-9dbb-9b2b20218538