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This allows the embedder to run the beforeunload handler, if any,
before closing a tab.

Bug: 1055540,1025256
Change-Id: I894256d0a05f11d58fa4234265beb03b34a019b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2072922
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745985}