Implement platform deleters per spec.

Extend the functionality of platform deleter operations slightly to
allow them to implement what WebIDL prescribes,

   http://heycam.github.io/webidl/#delete

That is, if the property name being deleted isn't exposed by the
platform object, any native property should be deleted instead.

Update the two deleters that this currently applies to, Storage and
DOMStringMap (dataset).

This change reflects the outcome of a spec clarification issue raised
via https://www.w3.org/Bugs/Public/show_bug.cgi?id=24096

R=
BUG=329176

Review URL: https://codereview.chromium.org/106853005

git-svn-id: svn://svn.chromium.org/blink/trunk@164081 bbb929c8-8fbe-4397-9dbb-9b2b20218538