Add comments explaining why the cast bots run two test suites unswarmed.

Also add a data dep that's necessary (but apparently not sufficient) for
cast_shell_browsertests to run on swarming.

Bug: 861753, 861843
Change-Id: Ib10a8589c1349ac70dc4d4358395af6b2867d16b
Reviewed-on: https://chromium-review.googlesource.com/1133201Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574156}

chromecast/browser/BUILD.gn

@@ -462,6 +462,10 @@ cast_source_set("browsertests") {
     "//components/prefs",
     "//media:test_support",
   ]
+
+  data = [
+    "//media/test/data/",
+  ]
 }
 
 cast_source_set("unittests") {

testing/buildbot/test_suite_exceptions.pyl

@@ -330,7 +330,7 @@
           '--test-launcher-jobs=1',
         ],
         'swarming': {
-          'can_use_on_swarming_builders': False,
+          'can_use_on_swarming_builders': False,  # https://crbug.com/861753
         },
       },
       'Cast Linux': {
@@ -341,7 +341,7 @@
           '--test-launcher-jobs=1',
         ],
         'swarming': {
-          'can_use_on_swarming_builders': False,
+          'can_use_on_swarming_builders': False,  # https://crbug.com/861753
         },
       },
     },
@@ -778,7 +778,7 @@
           '--test-launcher-filter-file=src/testing/buildbot/filters/cast-linux.content_browsertests.filter',
         ],
         'swarming': {
-          'can_use_on_swarming_builders': False,
+          'can_use_on_swarming_builders': False,  # https://crbug.com/861843
         },
       },
       'Cast Linux': {
@@ -786,7 +786,7 @@
           '--test-launcher-filter-file=src/testing/buildbot/filters/cast-linux.content_browsertests.filter',
         ],
         'swarming': {
-          'can_use_on_swarming_builders': False,
+          'can_use_on_swarming_builders': False,  # https://crbug.com/861843
         },
       },
       # chromium.memory