Move core/features.gypi to build/features.gypi

R=abarth@chromium.org, eseidel@chromium.org
BUG=297477

Review URL: https://codereview.chromium.org/26565004

git-svn-id: svn://svn.chromium.org/blink/trunk@159267 bbb929c8-8fbe-4397-9dbb-9b2b20218538