Remove multi-frame image decoder when the image is completely decoded


Count decoded frames in ImageFrameGenerator, and remove remove multi-frame image decoders when the image is completely decoded

This saves memory on pages with a lot of multi-frame images. data and discussion please see https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/KZ8EDtEZ9-c

BUG=429614

Review URL: https://codereview.chromium.org/688423004

git-svn-id: svn://svn.chromium.org/blink/trunk@185311 bbb929c8-8fbe-4397-9dbb-9b2b20218538