Remove structured cloning restriction over accessor properties.

The structured cloning algorithm,

 http://www.whatwg.org/specs/web-apps/current-work/#safe-passing-of-structured-data

treats accessor and non-accessor properties the same on an object
being cloned. For a short two month period, the spec'ed algorithm
imposed a stringent restriction on the cloning of accessor properties
-- no accessor property value could appear within/inside the value of
another accessor property. If any did, their values were mapped to
'null'; see

 https://www.w3.org/Bugs/Public/show_bug.cgi?id=12101

The structured cloning implementation provided this; a change that was
subsequently reverted and generalized to allow accessor properties to
be cloned without any side conditions on how they are nested. Follow
up on that spec change here and do the same. Other depth controls on
nested object structures are still effective.

R=mkwst
BUG=278883

Review URL: https://codereview.chromium.org/97883002

git-svn-id: svn://svn.chromium.org/blink/trunk@162969 bbb929c8-8fbe-4397-9dbb-9b2b20218538