Make dirty checking in check_gn_headers.py more robust

BUG=733898
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_dbg_ng

Review-Url: https://codereview.chromium.org/2943043002
Cr-Commit-Position: refs/heads/master@{#480233}


CrOS-Libchrome-Original-Commit: 67aabe00c117998937559c4847d85451abe8450b
1 file changed
tree: 2b0dbdb013a769d0c82fd5ab3a7dfcc003588f2a
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/