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This CL enters Chrome into VR mode upon receiving a VR intent. It facilitates
a smooth transition into VR by having VR intents go through VrMainActivity
which has the correct manifest flags to keep the system in VR mode and 
disabling the window preview feature to hide the system UI on startup. Note
that currently, VrMainActivity doesn't have any intent-filters and can only
be explicitly targeted.

VrMainActivity continues to open a CCT for intents from the Daydream app
and opens a CTA otherwise.

Follow-up work:
- Fix issues with devices that support multiple screen densities 

Note that this feature is guarded behind the "VrLaunchIntent" runtime 
flag.

Bug: 752200
Change-Id: I7656e0df53c0095ca3a958ede281e2ef7157cd95
Reviewed-on: https://chromium-review.googlesource.com/722456Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Yash Malik <ymalik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512877}