Convert file objects to strings, before passing to PresubmitNotifyResult

Json support, added in
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1666250,
breaks with object which aren't serializable.

Bug: chromium:971895
Change-Id: I36be5a548f8b82d4969fb38c34521e8df1a0b4c2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/143882
Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28384}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 7c423c8..2e3abc3 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -175,11 +175,11 @@
       if path == 'api':
         # Special case: Subdirectories included.
         if dn == 'api' or dn.startswith('api/'):
-          files.append(f)
+          files.append(f.LocalPath())
       else:
         # Normal case: Subdirectories not included.
         if dn == path:
-          files.append(f)
+          files.append(f.LocalPath())
 
   if files:
     return [output_api.PresubmitNotifyResult(API_CHANGE_MSG, files)]