WebAssembly.instantiate is polymorphic, it can either take a module
object as parameter, or a buffer source which should be compiled first.
To share code between the two implementations, the module object was
first passed to a promise (i.e. which is the result of compilation).
However, passing the module object to a promise has a side effect if
the module object has a then function. To avoid this side effect I
remove this code sharing and call AsyncInstantiate directly in case
the parameter is a module object.

R=mstarzinger@chromium.org

Bug: chromium:836141
Change-Id: I67b76d0d7761c5aeb2cf1deda45b6842e494eed4
Reviewed-on: https://chromium-review.googlesource.com/1025774Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52755}