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As of the time of this CL (M66), window's unload event doesn't not fire
when the user navigates away with the back button. To make the behavior
consistent between tab-closinng/reload/back, we instead listen to the
beforeunload event.

Bug: 806912
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I319683f14284e4d6884aa22c84c78fea08ac78ef
Reviewed-on: https://chromium-review.googlesource.com/965251
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543909}