Slim down generated Callback classes and enforce more at compile-time

Removed unused executionContext() virtual method which was being generated for
every callback class (it was previously used by MutationCallback, but that's
completely custom these days).

Also enforced that callbacks are always constructed with v8::Objects rather
than values, and added a FIXME to switch to v8::Function if we can change
WebSQL's behavior.

R=haraken@chromium.org, nbarth@chromium.org

Review URL: https://codereview.chromium.org/107343004

git-svn-id: svn://svn.chromium.org/blink/trunk@163455 bbb929c8-8fbe-4397-9dbb-9b2b20218538