Ignore the --gtest_output command line argument.

This is required in order to run the Telemetry based WebGL conformance
tests under runtest.py, which is in turn needed in order to properly
parse the test results and upload them to the flakiness dashboard.

Eventually Telemetry should ignore this argument rather than each
individual test, so that all Telemetry based tests can easily be run
on the bots, but that will require a full review, so doing this
workaround for now.

BUG=225939
TBR=bajones@chromium.org

Review URL: https://codereview.chromium.org/25399005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227045 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227045