Add threads stack dumping feature (through SIGUSR1) to test_runner.py.

BUG=320445
R=bulach@chromium.org

Review URL: https://codereview.chromium.org/59873011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235996 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235996