PRESUBMIT: don't check .md files for valid OS macros

BUG=

Review-Url: https://codereview.chromium.org/1957523002
Cr-Commit-Position: refs/heads/master@{#392065}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 29ae697e..6576b32b 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -1937,7 +1937,7 @@
   """Check all affected files for invalid OS macros."""
   bad_macros = []
   for f in input_api.AffectedFiles():
-    if not f.LocalPath().endswith(('.py', '.js', '.html', '.css')):
+    if not f.LocalPath().endswith(('.py', '.js', '.html', '.css', '.md')):
       bad_macros.extend(_CheckForInvalidOSMacrosInFile(input_api, f))
 
   if not bad_macros: