D3D12: Support creating compute pipeline asynchronously

This patch implements the asynchronous path of CreateComputePipelineAsync
on D3D12 backend with the basic framework of the dawn_unittest
AsyncTaskTest.Basic.

1. Call the constructor of dawn_native::d3d12::ComputePipeline in the main
   thread.
2. Execute dawn_native::ComputePipelineBase::Initialize() (a virtual function)
   asynchronously.
3. Ensure every operation in dawn_native::d3d12::ComputePipeline::Initialize()
   is thread-safe (PersistentCache).
4. Save all the return values (pipeline object or error message, userdata, etc)
   in a CreateComputePipelineAsyncWaitableCallbackTask object and insert this
   callback task into CallbackTaskManager.
5. In Callback.Finish():
- Insert the pipeline object into the pipeline cache if necessary
- Call WGPUCreateComputePipelineAsyncCallback

Note that as we always handle the front-end pipeline cache in the main thread,
we don't need to make it thread-safe right now.

BUG=dawn:529
TEST=dawn_end2end_tests

Change-Id: I7eba2ce550b32439a94b2a4d1aa7f1b3383aa514
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/47900
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/tests/unittests/AsyncTaskTests.cpp

@@ -67,7 +67,7 @@ TEST_F(AsyncTaskTest, Basic) {
     dawn_platform::Platform platform;
     std::unique_ptr<dawn_platform::WorkerTaskPool> pool = platform.CreateWorkerTaskPool();
 
-    dawn_native::AsnycTaskManager taskManager(pool.get());
+    dawn_native::AsyncTaskManager taskManager(pool.get());
     ConcurrentTaskResultQueue taskResultQueue;
 
     constexpr size_t kTaskCount = 4u;