telemetry: screenshot.py uses --file instead of --output

NOTRY=true

Review URL: https://codereview.chromium.org/66033005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234227 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234227