VR: Extract the WebXR presentation state in a separate file

Bug: 767282
Change-Id: Ie0a3951f454f8ee1d8df36d770b758d06d9a6826
Reviewed-on: https://chromium-review.googlesource.com/1157269
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Aldo Culquicondor <acondor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579791}
5 files changed