Have NodeChildRemovalTracker constructor take a reference

Have NodeChildRemovalTracker constructor take a reference instead of
a pointer and update NodeChildRemovalTracker::node() to return a
reference, as those pointers could not be null.

This makes the calling code look safer.

R=adamk

Review URL: https://codereview.chromium.org/53373002

git-svn-id: svn://svn.chromium.org/blink/trunk@160988 bbb929c8-8fbe-4397-9dbb-9b2b20218538