[Extern Generation] Add a presubmit script to check externs not being updated

extensions/common/api/PRESUBMIT.py

Index: extensions/common/api/PRESUBMIT.py
diff --git a/extensions/common/api/PRESUBMIT.py b/extensions/common/api/PRESUBMIT.py
new file mode 100644
index 0000000000000000000000000000000000000000..b74059fe4c2aa40d4916945518d8c54944c21559
--- /dev/null
+++ b/extensions/common/api/PRESUBMIT.py
@@ -0,0 +1,37 @@
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Chromium presubmit script for src/extensions/common.
+
+See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
+for more details on the presubmit API built into depot_tools.
+"""
+
+import sys
+
+
+def _CheckExterns(input_api, output_api):
+  original_sys_path = sys.path
+
+  try:
+    sys.path.append(input_api.PresubmitLocalPath())
+    from externs_checker import ExternsChecker
+  finally:
+    sys.path = original_sys_path
+
+  join = input_api.os_path.join
+  api_root = input_api.PresubmitLocalPath()
+  externs_root = join(api_root, '..', '..', '..', 'third_party',
+      'closure_compiler', 'externs')
+
+  api_pairs = {
+    join(api_root, 'bluetooth.idl'): join(externs_root, 'bluetooth.js'),
+    # TODO(rdevlin.cronin): Add more!
+  }
+
+  return ExternsChecker(input_api, output_api, api_pairs).RunChecks()
+
+
+def CheckChangeOnUpload(input_api, output_api):
+  return _CheckExterns(input_api, output_api)