Isolate extensions_unittests.

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 4a5ffab86db773387e4379407be5c6836d4aef47..23fb01913a8f791282bcdca41ac69ac71495434e 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -120,6 +120,9 @@
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -298,6 +301,9 @@
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -506,6 +512,9 @@
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -714,6 +723,9 @@
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -922,6 +934,9 @@
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {