Revert of Optimize check_gn_headers.py for speed (patchset #1 id:1 of https://codereview.chromium.org/2842513003/ )

Reason for revert:
This optimization was buggy and broke the ninja parsing.

Original issue's description:
> 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}
> Committed: https://chromium.googlesource.com/chromium/src/+/6b609383cf3669b0670b567249ff868ea4b26e43

TBR=dpranke@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=661774

Review-Url: https://codereview.chromium.org/2844623002
Cr-Commit-Position: refs/heads/master@{#467231}
2 files changed