Revert of Delete mediaPlaybackRequiresUserGesture (https://codereview.chromium.org/414543002/)

Reason for revert:
We're going to gather some data about how users react to autoplaying videos in order to decide whether to keep this restriction

Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178731

TBR=eseidel@chromium.org,philipj@opera.com
NOTREECHECKS=true
NOTRY=true
BUG=178297

Review URL: https://codereview.chromium.org/428263004

git-svn-id: svn://svn.chromium.org/blink/trunk@179369 bbb929c8-8fbe-4397-9dbb-9b2b20218538