Drop [LegacyImplementedInBaseClass] from ParentNode interface

Drop [LegacyImplementedInBaseClass] from ParentNode interface and add
functions in ParentNode namespace to forward calls to the ContainerNode
class.

R=haraken

Review URL: https://chromiumcodereview.appspot.com/23005010

git-svn-id: svn://svn.chromium.org/blink/trunk@156305 bbb929c8-8fbe-4397-9dbb-9b2b20218538