GYP related files went in gypfiles/, and the gni went in gni/ this
changes frees up the build/ directory name for Chromium's build/
directory.

BUG=angleproject:1569

Change-Id: I76fe343d569239c2732ba87986fcf7debc21d417
Reviewed-on: https://chromium-review.googlesource.com/403029
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>