Convert gfx_unittests 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 da8068f037eaa86b7cc03e9a15093f7c1097937a..e42a4e989d8782ce27f14fc3b6167081ba97e38e 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -99,6 +99,9 @@
         "test": "gcm_unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gfx_unittests"
       },
       {
@@ -346,6 +349,9 @@
         "test": "gcm_unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gfx_unittests"
       },
       {
@@ -718,6 +724,9 @@
         "test": "gcm_unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gfx_unittests"
       },
       {
@@ -1376,6 +1385,9 @@
         "test": "gcm_unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gfx_unittests"
       },
       {