Drop [LegacyImplementedInBaseClass] from ChildNode IDL interface

Drop [LegacyImplementedInBaseClass] from ChildNode IDL interface and move the
related API from Node to a new ChildNode class. All the callers of Node's
previousElementSibling() / nextElementSibling() were already previously updated
to use ElementTraversal instead for consistency.

R=haraken, adamk
BUG=344224
NOTRY=true

Review URL: https://codereview.chromium.org/168673002

git-svn-id: svn://svn.chromium.org/blink/trunk@167249 bbb929c8-8fbe-4397-9dbb-9b2b20218538