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
Mark softwarecompositing/video-page-visibility as crashing.

This patch marks virtual/softwarecompositing/video-page-visibility.html
as crashing on debug on all bots that this test is run on (Linux, Win).

There's no obvious regression candidate so I'm suppressing for now.

TBR=piman
NOTRY=true
BUG=344237

Review URL: https://codereview.chromium.org/169143002

git-svn-id: svn://svn.chromium.org/blink/trunk@167250 bbb929c8-8fbe-4397-9dbb-9b2b20218538