Revert "Rename buildflag_headers for consistency"

This reverts commit b3e42b746f309641620a68ca2e3df07185c5132e.

Reason for revert: I just had a failure in the nacl side of things that I believe is related to this. I'm going to revert.

Original change's description:
> Rename buildflag_headers for consistency
> 
> This updates content.
> 
> Targets go from foo_features to foo_buildflags
> And similarly the header now has buildflags in the name.
> 
> BUG=814485
> TEST=covered by build
> 
> Change-Id: I42c4e6d6fd102d923575a0a3eccf77ec3b0e3276
> Reviewed-on: https://chromium-review.googlesource.com/949091
> Commit-Queue: Scott Violet <sky@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#540919}

TBR=sky@chromium.org,dcheng@chromium.org

Change-Id: I9f6a9ef5b6299bf9656b24db5eed63bbfa474f49
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 814485
Reviewed-on: https://chromium-review.googlesource.com/949983
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540966}
42 files changed