chromeos: disables arc.ContainerMount

it's flaky.

BUG=1114752
TEST=test only change

Change-Id: Ie28ea8bc048d3286d748bace5b0c560bd866b440
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2347046
Commit-Queue: Scott Violet <sky@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796444}

chromeos/BUILD.gn

@@ -315,6 +315,9 @@ if (is_chromeos_device) {
 
       # crbug.com/1114654
       "crostini.Restart.artifact",
+
+      # crbug.com/1114752
+      "arc.ContainerMount",
     ]
   }