GitLab

In not-surfaceless mode, webxr_ may not be initialized yet at the time
we get an incoming SetWebVrMode(false) call which calls EndPresentation.
Add a check to avoid a crash in that scenario.

BUG=846102

Change-Id: Ib5cd0d2483de831c6690cb8e1d3dbf139da44778
Reviewed-on: https://chromium-review.googlesource.com/1102122Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567694}