Have FontBuilder::initForStyleResolve() take a Document reference

Have FontBuilder::initForStyleResolve() take a Document reference instead of
a pointer as it can never be NULL. This also avoids having to convert a
reference into a pointer at the call sites.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23498013

git-svn-id: svn://svn.chromium.org/blink/trunk@157048 bbb929c8-8fbe-4397-9dbb-9b2b20218538