Make DecodingImageGenerator use new onGetYUV8Planes() signature.

It now specifies the YUV color space being used (always JPEG
in this case).

Review URL: https://codereview.chromium.org/566743004

git-svn-id: svn://svn.chromium.org/blink/trunk@182177 bbb929c8-8fbe-4397-9dbb-9b2b20218538