Use Cr-Commit-Position instead of svn rev for chromium_revision

test_run_results.py used to extract svn_revision from git log to use
as chromium_revision property in test results. After git migration,
this naturally got stuck at last svn revision (291560). Let's
use Cr-Commit-Position instead.

BUG=415038

Review URL: https://codereview.chromium.org/580753002

git-svn-id: svn://svn.chromium.org/blink/trunk@182170 bbb929c8-8fbe-4397-9dbb-9b2b20218538