Convert gcm_unit_tests to run exclusively on Swarming

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 fc72df886088e3672962211bcf44f2587f784ef2..d25ebe2baae94fd21479e90f6115e0198f7a0346 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -126,6 +126,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {
@@ -322,6 +325,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {
@@ -551,6 +557,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {
@@ -780,6 +789,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {
@@ -1009,6 +1021,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {