This CL fixes a bad interleaving that can happen between the
CompilationState and streaming compilation. In that particular
interleaving, streaming compilation tries to restart compilation tasks
after compilation failed already, which fails with a check in the
CancelableTaskManager. The problem is the following:

The CompilationState notifies the streaming decoder of compilation
errors. After receiving the notification, the streaming decoder
ignores all subsequent incoming bytes. However, the CompilationState
does not notify the streaming decoder directly, it posts a task
which will notify the streaming decoder. This means that between the
failing of compilation and the notification of the streaming decoder
there is a time window in which compilation already failed but the
streaming decoder can still restart compilation tasks. The crash
happened when this time window was hit.

With this CL we check that compilation did not fail before we restart
compilation tasks. I was never able to reproduce this crash, so I don't
really know if this particular issue caused the crash.

R=titzer@chromium.org

Bug: chromium:840713
Change-Id: Ic522b1c21a7d7749c1e7f6097aa450a09fb271cc
Reviewed-on: https://chromium-review.googlesource.com/1075787Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53428}