DO NOT MERGE Support WebP Alpha (Lossless) images

Android/Skia supports pre-multiplied Alpha. WebP image format supports
Alpha via color modes MODE_RGBA & MODE_RGBA_4444 (corresponding ones for
premultiplied alpha are MODE_rgbA & MODE_rgbA_4444).
The pre-requisite change for this one is the libwebp patch:
https://googleplex-android-review.googlesource.com/#/c/219333/

Change-Id: I30b0d8d4b0325eb3c925371ac8f3a9b7d4801639
1 file changed