Added flag simplified-fullscreen-ui.

This flag will be used for experimenting with a streamlined fullscreen
and mouse pointer lock UX. In this patch, the flag removes the prompt to
allow sites to go fullscreen or take pointer lock (permission is
automatically granted).

BUG=352425

Review URL: https://codereview.chromium.org/1252503007

Cr-Commit-Position: refs/heads/master@{#341281}
9 files changed