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When chromedriver closes the last tab, it should still trigger
unload event handler before calling quit. The expected behavior is
after closing the last tab via close webview, browser should be able
to process the unload events. After calling quit, the Cookies should
have been saved properly in user-data-dir/Default/Cookies.

Bug: chromedriver:3370,chromedriver:3635
Change-Id: Id00eaa516026106d2704b193957d418a56e8945f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518211Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Shengfa Lin <shengfa@google.com>
Cr-Commit-Position: refs/heads/master@{#823772}