Convert gfx_unittests to run exclusively on Swarming

testing/buildbot/chromium.chromiumos.json

Index: testing/buildbot/chromium.chromiumos.json
diff --git a/testing/buildbot/chromium.chromiumos.json b/testing/buildbot/chromium.chromiumos.json
index 7ccfef887c779db6f0558d2c2d6ed7c625c2d13d..6d91453433d0dc296e9cd20843903433ec5acef4 100644
--- a/testing/buildbot/chromium.chromiumos.json
+++ b/testing/buildbot/chromium.chromiumos.json
@@ -210,6 +210,9 @@
         "test": "gcm_unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gfx_unittests"
       },
       {
@@ -408,6 +411,9 @@
         "test": "gcm_unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gfx_unittests"
       },
       {
@@ -600,6 +606,9 @@
         "test": "gcm_unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gfx_unittests"
       },
       {