Android: allow a --single-step for test_runner.py perf.

This is the src/ part of trying to make android up and downstream run more or
less the same stack for perf tests.
Instead of requiring all steps listed (which allows sharding), this instead
allows running a single step, with retries and other niceties such as adb
restarting, device fail over, etc..

BUG=

Review URL: https://codereview.chromium.org/66373004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234493 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234493