Have Document::fonts() return a raw pointer

Have Document::fonts() return a raw pointer instead of a PassRefPtr as
we are not passing ownership to the caller.

R=adamk
BUG=

Review URL: https://codereview.chromium.org/27544002

git-svn-id: svn://svn.chromium.org/blink/trunk@159846 bbb929c8-8fbe-4397-9dbb-9b2b20218538