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/+/2421884Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809154}

content/test/gpu/gpu_tests/common_browser_args.py

@@ -6,8 +6,7 @@
 AUTOPLAY_POLICY_NO_USER_GESTURE_REQUIRED =\
     '--autoplay-policy=no-user-gesture-required'
 DISABLE_ACCELERATED_2D_CANVAS = '--disable-accelerated-2d-canvas'
-DISABLE_DEVICE_DISCOVERY_NOTIFICATIONS =\
-    '--disable-device-discovery-notifications'
+DISABLE_DEVICE_DISCOVERY_NOTIFICATIONS = '--disable-features=MediaRouter'
 DISABLE_DOMAIN_BLOCKING_FOR_3D_APIS = '--disable-domain-blocking-for-3d-apis'
 DISABLE_FEATURES_D3D11_VIDEO_DECODER = '--disable-features=D3D11VideoDecoder'
 DISABLE_FORCE_FULL_DAMAGE =\