Mojo JS Bindings: cleanup stub class constructor usage

A couple of weeks ago, the constructor parameter for stub classes was
changed to be the implementation delegate[1]. It had been the proxy
instance. Cleaned up some old tests that relied on the old behavior.

[1] - https://codereview.chromium.org/742973002.

BUG=
R=aa@chromium.org

Review URL: https://codereview.chromium.org/750883003