Isolate extensions_unittests.

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 d32c10d3a4077d431698fa8d54cd522cbfd4acda..ba34a51938142d88e7fac056b7aff77464d8b90a 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -93,6 +93,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -346,6 +349,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -727,6 +733,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -988,6 +997,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {
@@ -1391,6 +1403,9 @@
         "test": "extensions_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "extensions_unittests"
       },
       {