Synced the WebP Code (Encoder/Decoder) with Head.

Added three color-spaces viz ARGB_8888, RGBA_4444, RGB_565 supported by
Android.
- Mentioned the WebP head change#Ia53f845b
  (https://review.webmproject.org/#change,2650)
- Changed the color-mode ARGB_4444 to RGBA_4444 (this is the actual one
  used in Skia).
- Fixed one bug (4 bit overflow) with RGBA_4444 mode.

Change-Id: I5536ec10176eae01316b09ba82a55dab6055de3d