Optimize check_gn_headers.py for speed

Process the output of 'ninja' live through the pipe rather than waiting
for it to finish.

Before:
20.74user 16.36system 0:24.57elapsed 151%CPU

After:
16.63user 9.24system 0:13.94elapsed 185%CPU

BUG=661774

Review-Url: https://codereview.chromium.org/2842513003
Cr-Commit-Position: refs/heads/master@{#466900}


CrOS-Libchrome-Original-Commit: 6b609383cf3669b0670b567249ff868ea4b26e43
2 files changed
tree: ef660138d9faff0489bd5bf64f2b72d733fdbed1
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/