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- CHECK (as opposed to DCHECK) this function isn't reentered.
- Check the RenderWidgetHost isn't deleted while iterating on its
  |observer_list|.

Bug: 1153966
Change-Id: I6ff4c6163fdd81126219009b8103dbe5008b9e9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584007
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835896}