Re-implement %Generator% intrinsic as an object

Re-implement %Generator% intrinsic as an object

From ES6 25.2.3 ("Properties of the GeneratorFunction Prototype
Object"):

> The GeneratorFunction prototype object is an ordinary object. It is
> not a function object and does not have an [[ECMAScriptCode]] internal
> slot or any other of the internal slots listed in Table 27 or Table
> 56.

Introduce one assertion for the value's type and additional tests for its
properties. Remove an invalid assertion that fails as a result of this
fix.

BUG=v8:3991
LOG=N

Review URL: https://codereview.chromium.org/1062633002

Cr-Commit-Position: refs/heads/master@{#27603}