This patch implements CallbackTask and CallbackTaskManager to store
the callbacks of Create*PipelineAsync().
In the futureCallbackTaskManager will manage all the callbacks that
should be called in Device.Tick().
BUG=dawn:529
Change-Id: I6ad4352371eb44515bc2d85cdc68220c9b758b8e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/49060
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
