Disable MediaRouterUIBrowserTest.EphemeralToolbarIconForDialog on Linux

The test seems to have been disabled on platforms *other than*
Windows/Linux/CrOS on accident. This change disables it just on Linux
where it's been flaky.

Bug: 1004635
Change-Id: If87e8999888006c0e52d796d6087c49c92460719
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2823096


Reviewed-by: Brandon Tolsch <btolsch@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#871965}