Have HTMLFrameOwnerElement / SubframeLoadingDisabler deal with references

Have HTMLFrameOwnerElement / SubframeLoadingDisabler deal with references
instead of pointers when possible (i.e. when the pointers are expected to
be non-null).

R=adamk

Review URL: https://codereview.chromium.org/53303002

git-svn-id: svn://svn.chromium.org/blink/trunk@160972 bbb929c8-8fbe-4397-9dbb-9b2b20218538