This makes it easier to implement asynchronous compilation by hiding all the implementation details of both synchronous and asynchronous compilation within wasm-module.cc, whereas before the code in wasm-js.cc actually implemented asynchronous compilation in terms of synchronous.

BUG=

Review-Url: https://codereview.chromium.org/2695813005
Cr-Commit-Position: refs/heads/master@{#43310}