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Measure time taken for VR UI steps to help catch regressions.
During WebVR/WebXR presentation, these times should ideally
be close to zero if there aren't any active overlays.

Also adds an overlay element counter, this should only be nonzero
during presentation for the beginning toast or temporary overlays
such as permission prompts.

Change-Id: Id6687bf82dc787f20b4bfd26763b1c3d63416d38
Reviewed-on: https://chromium-review.googlesource.com/998515
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548586}