[wasm] Refactor async compile tasks

Make each compile phase a separate task. This allows us to store
additional information in each task instead of "globally" on the
AsyncCompileJob.
This CL only does the initial refactoring, without changing where data
is actually stored.

R=ahaas@chromium.org, mtrofin@chromium.org

Change-Id: Iad73de7b7d09f716c527e6d241314fde52aa55f8
Reviewed-on: https://chromium-review.googlesource.com/488142
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44965}