If testfilter parameter is passed, put it in to the gerrit tryjob
as a property. Later it will be processed by GerritPoller on Buildbot
master.

R=victorhsieh@chromium.org
BUG=468075

Review URL: https://codereview.chromium.org/1019713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294500 0039d316-1c4b-4281-b951-d872f2087c98