When setting a generator function's  "prototype" property to a non-object, the
prototype of new generator instances should be %GeneratorPrototype%, not
%ObjectPrototype%.

R=verwaest@chromium.org
BUG=v8:5011

Review-Url: https://codereview.chromium.org/1982203003
Cr-Commit-Position: refs/heads/master@{#36313}