[Closure Externs] Add more APIs to the presubmit warning list

extensions/common/api/externs_checker.py

Index: extensions/common/api/externs_checker.py
diff --git a/extensions/common/api/externs_checker.py b/extensions/common/api/externs_checker.py
index 58c0076f8cec0378898334db76a6a0ba5156fb79..fd60fdd87df4901e4679f3d7e25845caf1d0523d 100644
--- a/extensions/common/api/externs_checker.py
+++ b/extensions/common/api/externs_checker.py
@@ -12,6 +12,10 @@ class ExternsChecker(object):
     self._output_api = output_api
     self._api_pairs = api_pairs
 
+    for path in api_pairs.keys() + api_pairs.values():
+      if not input_api.os_path.exists(path):
+        raise OSError('Path Not Found: %s' % path)
+
   def RunChecks(self):
     bad_files = []
     affected = [f.AbsoluteLocalPath() for f in self._input_api.AffectedFiles()]