Update GPU device discovery flag

Updates the flag used by the GPU Telemetry-based tests to disable
device discovery notifications. Previously, this was achieved via
--disable-device-disicovery-notifications, but is now achieved via
--disable-features=MediaRouter.

TBR=kbr@chromium.org

Change-Id: Ib83b88e0d77dce2745a3d57b31449b2dad1016ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421884
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809154}