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If a page has beforeunload/unload handlers (and soon, pagehide and
visibilitychange handlers too), tabs.remove might not be able to close
a tab immediately. We shouldn't trigger the callback passed to
tabs.remove until the tabs actually closed. This CL waits for
WebContentsDestroyed to be called on all the tabs that we're removing
before responding/triggering callbacks.

Bug: 987409
Change-Id: Ibdf526307c8c2652bc04a7aca4c379aca6d11922
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2319927
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794852}