Support setMediaPlaybackRequiresUserGesture in WebSettings.

Adds another WebSettings toggleable, along the lines of: https://github.com/robolectric/robolectric/commit/7eff08c896ccce4a01c887b89db64bcb808158e4

I'm making the pull request here because I need the 4.4 robolectric code, FWIW.