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Currently WebXr uses a custom consent flow. Due to changes in the spec,
and desired expansion to other platforms, WebXr needs to switch to using
the Permissions API instead. This change adds a flag to enable
development of switching the consent prompt out with calls to the
permissions api.  Note that while the spec is evolving to allow proper
querying by sites in the renderer, this flag is intended to only gate
switching the internals. (e.g. using the permissions API to prompt for
user consent to initiate a WebXr session).

Bug: 1033592
Change-Id: I0eec6308f6432cbff3c60bff3e949fe3b2205926
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992267
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729556}