Exclude base/message_loop files from MessageLoopProxy presubmit check

Until we completely stop using it we may still need to
touch the code inside message_loop.

BUG=465354

Review URL: https://codereview.chromium.org/1094263002

Cr-Commit-Position: refs/heads/master@{#326145}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 99c0260..ba715e1 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -257,7 +257,10 @@
         'Please use SingleThreadTaskRunner or ThreadTaskRunnerHandle instead.'
       ),
       True,
-      (),
+      (
+        # Internal message_loop related code may still use it.
+        r'^base[\\\/]message_loop[\\\/].*',
+      ),
     ),
 )