[es6] Iterators and generators should "extend" %IteratorPrototype%

[es6] Iterators and generators should "extend" %IteratorPrototype%

All the builtin iterators as well as the generator objects have an
object called %IteratorPrototype% in the spec between them and
%ObjectPrototype%.

BUG=v8:3568
LOG=N

Review URL: https://codereview.chromium.org/1128233008

Cr-Commit-Position: refs/heads/master@{#28426}