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 51e2394b6b243ad319fc1d32ca5a41848d580981..01b46a43cf02a61afda9436f447e75a03bd1a243 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"
       },
       {
@@ -310,6 +313,9 @@
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -533,6 +539,9 @@
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -756,6 +765,9 @@
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -979,6 +991,9 @@
         "test": "crypto_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {