The ScheduledErrorThrower is also needed in the wasm-async fuzzer so I
moved the implementation from wasm-js.cc to wasm-api.[h|cc].

R=clemensh@chromium.org

Bug: chromium:749838
Change-Id: I49d7438d1ec0281285ce0c64ba462c22001be08e
Reviewed-on: https://chromium-review.googlesource.com/591447
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47112}