Convert gcm_unit_tests to run exclusively on Swarming

testing/buildbot/chromium.win.json

Index: testing/buildbot/chromium.win.json
diff --git a/testing/buildbot/chromium.win.json b/testing/buildbot/chromium.win.json
index 11f5182d5d8d63b82ea22ec05a9f89daf42a8add..fd721410ad261c50f0bf2bd96a1b92052c5569f3 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -102,6 +102,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {
@@ -382,6 +385,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {
@@ -790,6 +796,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {
@@ -1079,6 +1088,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {
@@ -1509,6 +1521,9 @@
         "test": "extensions_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gcm_unit_tests"
       },
       {