Make some arguments of FocusController functions references.

We can remove unnecessary assertions and runtime null checks by this change.

BUG=
TEST=none; No behavior changes.

Review URL: https://codereview.chromium.org/704963002

git-svn-id: svn://svn.chromium.org/blink/trunk@184893 bbb929c8-8fbe-4397-9dbb-9b2b20218538