Animated WebP decoder: rectify post-processing when disposing to background.

See the corresponding (correct) logic on Chrome side:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/platform/image-decoders/webp/WEBPImageDecoder.cpp&l=446

Change-Id: Ife17e24b3263ff6148ffc38849c5e3527d341945
1 file changed