Rename buildflag_header targets in base for consistency

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: Ib936c2840991902fa33c040b381b8ffc98a3c532
Reviewed-on: https://chromium-review.googlesource.com/930033
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#538310}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4416579b311cc64b476335b5d46ddbfa6131ed0e