Convert gl_unittests 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 8000df44c1b6a6b479830456aa4f67e9f2b9b23a..0afd311c4bb1c60c40e1042a204eb3e09a06d269 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -349,6 +349,9 @@
         "test": "gfx_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gl_unittests"
       },
       {
@@ -593,6 +596,9 @@
         "test": "gfx_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gl_unittests"
       },
       {
@@ -837,6 +843,9 @@
         "test": "gfx_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gl_unittests"
       },
       {
@@ -1081,6 +1090,9 @@
         "test": "gfx_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gl_unittests"
       },
       {