DevTools: browser.close should always terminate browser

This patch teaches Browser.close protocol method to terminate
browser regardless of weather it has beforeunload dialogs or
pending downloads.

R=dgozman

Change-Id: I1900c35fc159ed9cb1cd717af99605c2fd84e2d7
Reviewed-on: https://chromium-review.googlesource.com/c/1381513
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618425}
6 files changed