Have ChildFrameDisconnector / ChildListMutationScope deal with references

Have ChildFrameDisconnector / ChildListMutationScope deal with references instead
of pointers as these could never be null.

R=adamk

Review URL: https://codereview.chromium.org/51273002

git-svn-id: svn://svn.chromium.org/blink/trunk@160877 bbb929c8-8fbe-4397-9dbb-9b2b20218538