Add tests for Android's layout test driver's tombstone method.

This is a frequent source of flakiness but isn't covered by any tests.
Introduce tests for the current logic, making it easier to add new tests
in case of future problems.

Also fixes a problem in _get_last_stacktrace where the format of the
first tombstone file is invalid, in which case we'd still crash.

R=dpranke, johnme
BUG=

Review URL: https://codereview.chromium.org/76423002

git-svn-id: svn://svn.chromium.org/blink/trunk@162914 bbb929c8-8fbe-4397-9dbb-9b2b20218538