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Currently, chrome's implementation of Target.disposeBrowserContext()
fails to dispose empty browser contexts.

We don't need to wait for browsers to close if there are no opened
browser windows to destroy a browser context.

R=dgozman

Change-Id: I7b41a1b55b398f543a547fd6ead9f83af7f8a843
Reviewed-on: https://chromium-review.googlesource.com/1050909Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557085}