Remove the HTMLVideoElement-specific prefixed fullscreen API

Intent to Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/Bxe7DnDVRZ0/5K61HQPrNK4J

BUG=346236

Review URL: https://codereview.chromium.org/178003008

git-svn-id: svn://svn.chromium.org/blink/trunk@167815 bbb929c8-8fbe-4397-9dbb-9b2b20218538