GitLab

Prevents a page from reading or resetting the pose of a
device which is currently being presented to by a
different page, as that potentially creates an
information leak between pages.

Also prevented pages from beginning presentation if another page is already presenting to a device. This is a
little overly aggressive right now, as disallows
presentation to more than one device at a time from any
page, but that's a reasonable limitation at this point
given the rarity of VR hardware.

BUG=389343

Review-Url: https://codereview.chromium.org/2329893002
Cr-Commit-Position: refs/heads/master@{#418771}