Rename FocusDirection to FocusType.

FocusDirection has some non-directional items. Focus*Direction* is not
appropriate.

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/168963002

git-svn-id: svn://svn.chromium.org/blink/trunk@167270 bbb929c8-8fbe-4397-9dbb-9b2b20218538