Enforce GN formatting for *.typemap files.

Ensure consistent formatting for Mojo typemap files that define the
build rules to generate StructTraits.

BUG=620799
TEST=Tested locally, both presubmit and git cl format work as expected.

Review-Url: https://codereview.chromium.org/2074793002
diff --git a/git_cl.py b/git_cl.py
index 75be64e..672d884 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -4778,7 +4778,7 @@
 def CMDformat(parser, args):
   """Runs auto-formatting tools (clang-format etc.) on the diff."""
   CLANG_EXTS = ['.cc', '.cpp', '.h', '.mm', '.proto', '.java']
-  GN_EXTS = ['.gn', '.gni']
+  GN_EXTS = ['.gn', '.gni', '.typemap']
   parser.add_option('--full', action='store_true',
                     help='Reformat the full content of all touched files')
   parser.add_option('--dry-run', action='store_true',