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It exists one RenderWidgetHostObserver deleting itself without calling
RemoveObserver(this) on its RenderWidgetHost.

All the observers are immune to this error, because they are using
base::ScopedObserver. All except RenderWidgetHostVisibilityTracker.

This class looks error free, but it doesn't hurt to make it use
base::ScopedObserver as well, just in case.

Bug: 1153966
Change-Id: I6160da2853d94cf647cfa55d79100ec6f52bd42b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594772
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837648}