gn: gyp_flag_compare.py, don't return non-zero exit code

Going to add it as a build step:
https://codereview.chromium.org/575733003/
but for now, don't want to error out.

TBR=brettw@chromium.org
BUG=335824

Review URL: https://codereview.chromium.org/577723002

Cr-Commit-Position: refs/heads/master@{#295131}

tools/gn/bin/gyp_flag_compare.py

@@ -214,7 +214,8 @@ def main():
     print diff
 
   print 'Total differences:', g_total_differences
-  return 1 if files_with_given_differences or different_source_list else 0
+  # TODO(scottmg): Return failure on difference once we're closer to identical.
+  return 0
 
 
 if __name__ == '__main__':