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When --no-startup-window is passed in combination with remote debugging,
browser is controlled by automation software. In this case, we keep
the application running until explicit Browser.close call over protocol.
Otherwise, application just exits because no browser window is opened.

Drive-by: apparently, this introduced a first test with DevToolsPipeHandler,
which exposed a DCHECK. Shutting down the pipe handler posts a task to
delete threads which joins them and requires sync primitives task trait.

Bug: 1054831
Change-Id: I1fcfdad11ec10661a2163b18a7547668655f25bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068043Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743967}