Convert chromedriver_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 ef1dfb2d38bd6ce2e533f2b1ada3e2385780664c..0cd906f9091a93913713c18941c9774708886fde 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -108,6 +108,9 @@
         "test": "cc_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "chromedriver_unittests"
       },
       {
@@ -301,6 +304,9 @@
         "test": "cc_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "chromedriver_unittests"
       },
       {
@@ -557,6 +563,9 @@
         "test": "cc_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "chromedriver_unittests"
       },
       {
@@ -807,6 +816,9 @@
         "test": "cc_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "chromedriver_unittests"
       },
       {
@@ -1057,6 +1069,9 @@
         "test": "cc_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "chromedriver_unittests"
       },
       {