Have Frame::navigationScheduler() return a reference

Have Frame::navigationScheduler() return a reference instead of a pointer
as it can never return null. This makes the calling code look safer.

Review URL: https://codereview.chromium.org/32793003

git-svn-id: svn://svn.chromium.org/blink/trunk@160270 bbb929c8-8fbe-4397-9dbb-9b2b20218538