Delete mediaPlaybackRequiresUserGesture

The Android framework does not require a user gesture to begin media playback.
The web shouldn't have that restriction either.

BUG=178297

Review URL: https://codereview.chromium.org/414543002

git-svn-id: svn://svn.chromium.org/blink/trunk@178731 bbb929c8-8fbe-4397-9dbb-9b2b20218538