Fix WebVR-to-UI handoff for permission prompts

The current implementation had an invalid DCHECK_EQ, there's a valid flow
where is_webvr_frame could be true while ShouldDrawWebVr was false. Remove
the invalid check and update the comment with a more accurate lifecycle
description.

Change-Id: I4eb1c7496972334c4d3b8e42fb3ffb352c0ed236
Reviewed-on: https://chromium-review.googlesource.com/1124994
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572323}