Make Document::nodeChildrenWillBeRemoved() take a reference

Make Document::nodeChildrenWillBeRemoved() take a reference instead of a
pointer as the pointer was always expected to be non-null. This makes the
code look safer and avoids having assertions.

R=adamk

Review URL: https://codereview.chromium.org/178613003

git-svn-id: svn://svn.chromium.org/blink/trunk@167734 bbb929c8-8fbe-4397-9dbb-9b2b20218538