There was a data race in the access of the foreground_task_runner_map_.
I protect each access to foreground_task_runner_map_ with a lock now.

Original change's description:
> [platform] Implement TaskRunners in the DefaultPlatform
>
> This CL implements the TaskRunners in the DefaultPlatform which has been
> added recently to the platform API. In addition I changed how task
> posting works on the DefaultPlatform.
>
> With this implementation the DefaultPlatform keeps one
> DefaultForegroundTaskRunner per isolate, plus one
> DefaultBackgroundTaskRunner. The DefaultPlatform owns these TaskRunners
> with a shared_ptr, which is also shared with any caller of
> GetForegroundTaskRunner or GetBackgroundTaskrunner.
>
> This CL moves the task management from the DefaultPlatform to the
> TaskRunners.  The DefaultForegroundTaskRunner owns and manages the the
> task queue, the delayed task  queue, and the idle task queue. The
> DefaultBackgroundTaskRunner owns the WorkerThread pool and the
> background task queue.
>
> In addition changed many Task* to std::unique_ptr<Task> to document task
> ownership.
>
> R=rmcilroy@chromium.org
>
> Change-Id: Ib9a01f1f45e5b48844a37d801f884210ec3f6c27
> Reviewed-on: https://chromium-review.googlesource.com/753583
> Commit-Queue: Andreas Haas <ahaas@chromium.org>
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49354}

Change-Id: Iddccdb07bde1a799815ec6ed6af37082df4987c7
Reviewed-on: https://chromium-review.googlesource.com/770970
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49379}