Rename buildflag_headers for consistency

This updates chrome/browser/vr

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: I89654d8c4db03d86bb96c63b18dfd0bd89b5be3c
Reviewed-on: https://chromium-review.googlesource.com/c/1403515
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621728}
3 files changed