When instantiating the wasm interpreter for debugging, we unwrap all
wasm-to-js wrappers and store the callable objects. The handles are
stored in a DeferredHandleScope and deleted when the InterpreterHandle
(store in WasmDebugInfo) is freed.
A call to an imported function reads the arguments from the stack,
converts them to JS objects, calls the callable, converts back the
return value and pushes it onto the stack.
Reentering the interpreter from the calles JS code is not permitted
yet, but will be in a follow-up CL.
Also, indirect calls to imported functions will have to follow.

R=titzer@chromium.org, ahaas@chromium.org
BUG=v8:5822

Change-Id: I66c35053bccb6cf8d416606e4f840d888ccb3b65
Reviewed-on: https://chromium-review.googlesource.com/453838
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43855}