Async compilation relies on background tasks, which are not available
in single-threaded mode.

Change-Id: Id47e7f5bd1406a5323d6a770936a23b6839e7936
Reviewed-on: https://chromium-review.googlesource.com/880583Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50789}