Initialize desktop VRDevices on first successful RequestSession

Prior to this change, navigating to a page that uses WebXR or WebVR APIs would
automatically start the runtimes, triggering windows to show and some wasted
resources.  After this change, a website needs to request a session for the
runtimes to start.

BUG=911329

Change-Id: Ieb039a36bb98e3af83793fff1f4884bacad8f824
Reviewed-on: https://chromium-review.googlesource.com/c/1359467
Commit-Queue: Bill Orr <billorr@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620809}
15 files changed