Add a UseCounter for the Attr.value setter with associated Element

This will distinguish between setting the value on a newly created Attr
object from Document.createAttribute() and setting the value on an Attr
object already attached to an Element, e.g. one from Element.attributes.

If the first case is common but the second is rare it won't be possible
to make value readonly, but it could still be possible to drop the
Element association, which is the point of making it readonly.

BUG=none

Review URL: https://codereview.chromium.org/208333014

git-svn-id: svn://svn.chromium.org/blink/trunk@169844 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce HeapSnapshotItem interface

The interface describes common part of nodes as well as containment and retainment edges. It used to iterate over the elements and serialize them.

BUG=None

Review URL: https://codereview.chromium.org/209653003

git-svn-id: svn://svn.chromium.org/blink/trunk@169845 bbb929c8-8fbe-4397-9dbb-9b2b20218538