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Ensure that magic window sessions get disconnected when VRServiceImpl is
destroyed. The default destructors don't do so, and this resulted in ID
collisions when the metrics helper is reused across page navigation.

Also add DVLOG(2) lifecycle logging to help with debugging in case we
run into similar issues in the future, the initialization sequence
is a bit complicated.

Bug: 1017959
Change-Id: Ib7876ec10c0db7076b599ec212762cb455ec5c0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884373
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710012}