Revert 162065 "Make generation IDs of animated GIF frames unique."

Very likely broke webkit_unit_tests destroyAllDecodedData/destroyDecodedDataExceptCurrentFrame on XP.

> 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

TBR=ernstm@chromium.org

Review URL: https://codereview.chromium.org/74453002

git-svn-id: svn://svn.chromium.org/blink/trunk@162140 bbb929c8-8fbe-4397-9dbb-9b2b20218538