Rename buildflag_headers for consistency

This renames most of components. There are still a couple left, but
this patch started getting big.

Targets go from foo_features to foo_buildflags
And similarly the header now has buildflags in the name.

BUG=814485
TEST=covered by build
TBR=tsepez@chromium.org

Change-Id: I321403895df9ddc988491c70a78507ec51434783
Reviewed-on: https://chromium-review.googlesource.com/943600


Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540227}