With this CL all kinds of Callable can imported into wasm. Please take a special look at the context that is used now in the WasmToJSWrapper.

BUG=633895
TEST=mjsunit/wasm/ffi.js

Review-Url: https://codereview.chromium.org/2208703002
Cr-Commit-Position: refs/heads/master@{#38569}