Refactoring: Page::focusController() should return FocusController&.

focusController() never be NULL because
 - m_focusController is owned by a Page,
 - it's initialized in the Page constructor.
 - It's not updated until it's destructed with the owner Page object.

Some redundant code is removed.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22707008

git-svn-id: svn://svn.chromium.org/blink/trunk@155919 bbb929c8-8fbe-4397-9dbb-9b2b20218538