Add extensions_unittests target to ChromiumOS testers.

This should make it run in the ChromiumOS config on the Main Waterfall.

BUG=399315
TEST=None
R=sky@chromium.org

Review URL: https://codereview.chromium.org/437693002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287465 0039d316-1c4b-4281-b951-d872f2087c98

testing/buildbot/chromium.chromiumos.json

@@ -9,6 +9,7 @@
       "crypto_unittests",
       "dbus_unittests",
       "display_unittests",
+      "extensions_unittests",
       "gcm_unit_tests",
       "google_apis_unittests",
       "gpu_unittests",
@@ -58,6 +59,7 @@
       "crypto_unittests",
       "dbus_unittests",
       "display_unittests",
+      "extensions_unittests",
       "gcm_unit_tests",
       "google_apis_unittests",
       "gpu_unittests",
@@ -101,6 +103,7 @@
       "components_unittests",
       "crypto_unittests",
       "dbus_unittests",
+      "extensions_unittests",
       "google_apis_unittests",
       "gpu_unittests",
       "url_unittests",