Background task restarting happens now in the CompilationState and not
in the AsyncCompileJob. The code in the AsyncCompileJob is dead, so I
remove it.

R=titzer@chromium.org

Change-Id: Ife52522a00ec43aa0f9ad50f8e0114c1e4da5db7
Reviewed-on: https://chromium-review.googlesource.com/978322Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52190}