Wait for all chrome.exe processes to exit before cleaning up.

test_chrome_with_chromedriver.py sometimes fails to delete the user data
dir because lingering chrome.exe processes (e.g., the crashpad handler)
are still working in it. This change causes the test to wait for all
Chrome procs to exit before searching for crash reports and cleaning up.

BUG=907979

Change-Id: I6500973a906a7b71b36d44aae4c1ca50d42f2001
Reviewed-on: https://chromium-review.googlesource.com/c/1352177
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611616}