Have Position deal with more references

Have Position deal with more references instead of pointers. This makes the
code look safer.

R=adamk

Review URL: https://codereview.chromium.org/188033005

git-svn-id: svn://svn.chromium.org/blink/trunk@168675 bbb929c8-8fbe-4397-9dbb-9b2b20218538