Convert printing_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 dc0d4e2beb9f157e22a6a72e278c37c0a97b3666..3bb13607b6ef5e549e6b1b25b4e52a450ded4deb 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -183,6 +183,9 @@
         "test": "ppapi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "printing_unittests"
       },
       {
@@ -391,6 +394,9 @@
         "test": "ppapi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "printing_unittests"
       },
       {
@@ -608,6 +614,9 @@
         "test": "ppapi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "printing_unittests"
       },
       {
@@ -825,6 +834,9 @@
         "test": "ppapi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "printing_unittests"
       },
       {
@@ -1043,6 +1055,9 @@
         "test": "ppapi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "printing_unittests"
       },
       {