GitLab

This change shares a single XrInstance between the XRRuntimeProvider,
OpenXrStatics, OpenXrDevice, OpenXrRenderLoop, and OpenXrApiWrapper.
The instance is actually owned by the OpenXrStatics, which is in turn
owned by the OpenXrDevice when one exists, and the XRRuntimeProvider
otherwise.

In terms of thead safety, the XrInstance will only be destroyed by the
OpenXrDevice destructor, which also owns the OpenXrRenderLoop and
explicitly shuts it down. Most methods which take an instance don't have
any threading concerns, but even so once we have a render loop we don't
make any calls against the instance from the OpenXrDevice thread until
the render loop is shut down.

Bug: 1100577
Change-Id: I2fc326e17c1faf3915e8eafddf23055ccd9a5639
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2278534
Commit-Queue: Brandon Jones <bajones@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Patrick To <patrto@microsoft.com>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786174}