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This enable Skia to decode WebP images. Since BitmapFactory relies
on Skia to decode images, this aims at adding support for WebP format
in any application relying on BitmapFactory. Decoder relies on
libwepb library, imported as external library in external/webp in
Android source tree.

Change-Id: I13ef4c1404b989af6a856e9150c62e297200331b