Synchronous compilation currently continues creating new tasks even
though compilation has already failed. This stops the creation of
new background tasks and makes sure that the background task manager
in the CompilationState is not canceled twice.

Change-Id: Ic4c55275ff70e7eca901ad357253f81aa8e2e8e1
Reviewed-on: https://chromium-review.googlesource.com/968781
Commit-Queue: Kim-Anh Tran <kimanh@google.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52044}