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The previous patch https://codereview.chromium.org/584513004 ran on all
metro viewer shutdowns, which would lead to Alt-F4 not being processed
which means that the proper metro process shutdown wouldn't happen,
potentially leaving the "ghost" icon.

Instead, only do the aggressive termination from the process_host side
and only when running test binaries (where we're concerned about having
the viewer be able to run again immediately.)

R=ananta@chromium.org
BUG=411147,416356

Review URL: https://codereview.chromium.org/584213002

Cr-Commit-Position: refs/heads/master@{#295995}