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This patch fixes a bug where permission prompts that were hidden due to
tab switching wouldn't be cleared upon navigation/destruction, leading
to in some cases a DCHECK being hit or prompts persisting across navs.

Bug: 757394
Change-Id: Iae960fb241ed635bd4eea93cd21cc50657aa4a9e
Reviewed-on: https://chromium-review.googlesource.com/627349Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496617}