Hook up "--threads" to gn help.

This was documented but the flag was not hooked up to the help system so it didn't actually appear in "gn help switches".

Review URL: https://codereview.chromium.org/1357523003

Cr-Original-Commit-Position: refs/heads/master@{#349500}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 62ce4f0ee6778343f52682294dc63784b02a7907
1 file changed
tree: 56ef200e35c4eea5ec08bca7337b6b6df817b3b1
  1. tools/