This adds code (behind a flag) to allow WebXr sessions to determine the
needed permissions and prompt the user for them on both Desktop and
Android.

In order to properly link to the PermissionManager and related classes,
VRServiceImpl and the classes that call into it needed to be moved into
the chrome/browser Build target.  Given that this target depends on
vr_common (where these other classes are), vr_common couldn't depend on
it, or else it would introduce a circular dependency. Therefore, these
files were moved into chrome/browser's target, which seemed to be common
for other classes that relied on PermissionManager.

vr_headset.png's generated from source SVG in crbug.com/912203 (which is
the source of the currently checked-in vr_headset.icon)

Note that strings are currently placeholders.

Showing the permission status in the page info bubble and site settings
will be done in future changes.

Bug: 1033592
Change-Id: I5be7f8b6e62c00fcb11ddeb6c2410720ec9c1bf0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992508
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731726}