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QuickOffice uses beforeunload events. When running inside a mime
handler, these beforeunload event handlers are ignored. Add a
beforeunload event handler to PluginDocument when a plugin requires it.

This event handler is controlled over a mojo interface, exposed to the
mime handler as chrome.mimeHandlerPrivate.setShowBeforeUnloadDialog().

Bug: 819761
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ieb61cf98ce2bf719fc128499f8d0e57d8fe7ad33
Reviewed-on: https://chromium-review.googlesource.com/1139946
Commit-Queue: Sam McNally <sammc@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576283}