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When we call -[NSApp terminate] in the app shim process, this does
not immediately terminate. Rather, it
 0. Sends the message appShimController_->host()->QuitApp() and waits
    for a reply before actually terminating.
 1. In the browser, this call hits ExtensionAppShimHandler::
    CloseBrowsersForApp
 2. Which will end up calling NativeWidgetMac::Close
 3. Which will then politely request that the app shim process close
    the corresponding window via
    views_bridge_mac::mojom::BridgedNativeWidget::CloseWindow
 4. Which will send an ack back to the browser via
    views_bridge_mac::mojom::BridgedNativeWidgetHost::OnWindowHasClosed
 5. Whereupon the browser window will be registered as closed, and a
    close message will be sent to the app shim, and the wait for
    termination will break

This ends up not working at step 3. Our suspended terminate ends up
running a nested message loop inside a mojo message handler, and so
it will never execute the CloseWindow message, and therefore never
close.

Change the logic at [0] to send the QuitApp message, and then
immediately terminate the application.

Bug: 902583
Change-Id: I60fd7b5fdf9f145369d06219e8ebef36cdcd0f44
Reviewed-on: https://chromium-review.googlesource.com/c/1364220Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614922}