Today, the semantics of:

WebAssembly.instantiate

and

WebAssembly.compile().then(new WebAssemblyInstance)

are subtly different, to the point where attempting the proposed
change uncovered bugs.

In the future, it's possible that .instantiate actually have different
semantics - if we pre-specialized to the provided ffi, for example.
Right now that's not the case.

This CL:
- gets our implementation closer to what developers may write using
the compile -> new Instance alternative, in particular wrt promise
creation. By reusing code paths, we uncover more bugs, and keep
maintenance cost lower.

- it gives us the response-based WebAssembly.instantiate implicitly.
Otherwise, we'd need that same implementation on the blink side. The
negative is maintenance: imagine if the bugs I mentioned could only be
found when running in Blink.

BUG=chromium:697028

Review-Url: https://codereview.chromium.org/2806073002
Cr-Commit-Position: refs/heads/master@{#44592}