Disable a unit test on Chromecast builds.

Unit test BaselinePolicy.SystemEperm in sandbox_linux_unittests fails
on internal build infra and it blocks some CLs.

Bug: b/148974697
Test: Internal CQ.
Change-Id: Ieb8f355910f46a7aae4ab4a42e8d1e7fbf4eb2a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063537
Commit-Queue: Guohui Deng <guohuideng@chromium.org>
Commit-Queue: Luke Halliwell (slow) <halliwell@chromium.org>
Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742722}

chromecast/BUILD.gn

@@ -108,6 +108,14 @@ cast_test_group("cast_tests") {
       "//components/exo:exo_unittests",
       "//sandbox/linux:sandbox_linux_unittests",
     ]
+
+    sandbox_linux_unittests_filter = {
+      test_name = "sandbox_linux_unittests"
+
+      # TODO(b/148974697): re-enable this.
+      gtest_excludes = [ "BaselinePolicy.SystemEperm" ]
+    }
+    filters += [ sandbox_linux_unittests_filter ]
   }
 
   if (is_linux || is_fuchsia) {