Initial support for the desktop media pipeline on Android.

Add experimental support for the desktop src= media pipeline on Android
behind a compile-time variable. To compile with it
enabled, set the gyp variable media_use_ffmpeg=1.

Currently only H/W accelerated VP8 is supported because that is all that
the existing AndroidVideoDecodeAccelerator supports.

BUG=507834

Review URL: https://codereview.chromium.org/1230793009

Cr-Commit-Position: refs/heads/master@{#339498}
10 files changed