[Telemetry] Pass --disable-gpu in run_tests if starting Xvfb

This CL works around issues with Mesa spinning up processes too early on Linux,
which causes browser tests to fail with the current reference build.

BUG=catapult:#3074

Review-Url: https://codereview.chromium.org/2573563002

telemetry/telemetry/testing/run_tests.py

@@ -89,6 +89,10 @@ def ProcessCommandLineArgs(cls, parser, args, _):
 
     if args.start_xvfb and xvfb.ShouldStartXvfb():
       cls.xvfb_process = xvfb.StartXvfb()
+      # Work around Mesa issues on Linux. See
+      # https://github.com/catapult-project/catapult/issues/3074
+      args.browser_options.AppendExtraBrowserArgs('--disable-gpu')
+
     try:
       possible_browser = browser_finder.FindBrowser(args)
     except browser_finder_exceptions.BrowserFinderException, ex: