Update from https://crrev.com/318214

testing/buildbot/chromium.mac.json

Index: testing/buildbot/chromium.mac.json
diff --git a/testing/buildbot/chromium.mac.json b/testing/buildbot/chromium.mac.json
index d7f39bb17a9504e6ef11a3e0eea914579309d43c..a5de3756f81c1a792a3168504a911a941dc09143 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -27,7 +27,12 @@
         }
       },
       "cacheinvalidation_unittests",
-      "cast_unittests",
+      {
+        "test": "cast_unittests",
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        }
+      },
       {
         "test": "cc_unittests",
         "swarming": {
@@ -183,7 +188,12 @@
         }
       },
       "cacheinvalidation_unittests",
-      "cast_unittests",
+      {
+        "test": "cast_unittests",
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        }
+      },
       {
         "test": "cc_unittests",
         "swarming": {
@@ -333,7 +343,12 @@
         }
       },
       "cacheinvalidation_unittests",
-      "cast_unittests",
+      {
+        "test": "cast_unittests",
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        }
+      },
       {
         "test": "cc_unittests",
         "swarming": {
@@ -483,7 +498,12 @@
         }
       },
       "cacheinvalidation_unittests",
-      "cast_unittests",
+      {
+        "test": "cast_unittests",
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        }
+      },
       {
         "test": "cc_unittests",
         "swarming": {