Add ProcessLifetimeObserver::OnTimedOut.

This allows consumers of TestLauncher and content::LaunchTests to take
action on child process timeout before the process is terminated.

BUG=764415
R=phajdan.jr@chromium.org,sky@chromium.org

Change-Id: Iccb359273c464f1e660a079af92edfc7feed817f
Reviewed-on: https://chromium-review.googlesource.com/746701
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513091}

• base/test/launcher/test_launcher.cc[diff]
• base/test/launcher/test_launcher.h[diff]
• content/public/test/test_launcher.cc[diff]
• content/public/test/test_launcher.h[diff]