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}