Have Frame::loader() return a reference

Have Frame::loader() return a reference instead of a pointer as it can never
return null. This makes the calling code look safer and gets rid of several
unnecessary null checks.

BUG=

Review URL: https://codereview.chromium.org/31063004

git-svn-id: svn://svn.chromium.org/blink/trunk@160365 bbb929c8-8fbe-4397-9dbb-9b2b20218538