Isolate extensions_unittests.

extensions/extensions_tests.gyp

 {
   'includes': [
     'extensions_tests.gypi',
   ],
   'variables': {
     # Product name is used for Mac bundle.
     'app_shell_product_name': 'App Shell',
     # The version is high enough to be supported by Omaha (at least 31)
     # but fake enough to be obviously not a Chrome release.
     'app_shell_version': '38.1234.5678.9',
@@ skipping 108 matching lines @@
           'dependencies': [
             'extensions_browsertests',
           ],
           'includes': [
             '../build/isolate.gypi',
           ],
           'sources': [
             'extensions_browsertests.isolate',
           ],
         },
+        {
+          'target_name': 'extensions_unittests_run',
+          'type': 'none',
+          'dependencies': [
+            'extensions_unittests',
+          ],
+          'includes': [
+            '../build/isolate.gypi',
+          ],
+          'sources': [
+            'extensions_unittests.isolate',
+          ],
+        }
       ],
     }],
   ],
 }