This mechanism ensures APIs like wasm async complete their work, 
without requiring use of natives (%APIs).

The mechanism is similar to the one used in content_shell,
which should allow us to easily port tests in that environment.

Review-Url: https://codereview.chromium.org/2842843005
Cr-Original-Commit-Position: refs/heads/master@{#44908}
Bug: 
Change-Id: I9deee0d256a600c60b42902fc8ef8478e5546344
Reviewed-on: https://chromium-review.googlesource.com/494968
Commit-Queue: Mircea Trofin <mtrofin@google.com>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45165}