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/dawn_native/Instance.cpp

@@ -18,6 +18,7 @@
 #include "common/Log.h"
 #include "dawn_native/ErrorData.h"
 #include "dawn_native/Surface.h"
+#include "dawn_platform/DawnPlatform.h"
 
 #if defined(DAWN_USE_X11)
 #    include "dawn_native/XlibXcbFunctions.h"
@@ -64,6 +65,7 @@ namespace dawn_native {
         return instance.Detach();
     }
 
+    // TODO(crbug.com/dawn/832): make the platform an initialization parameter of the instance.
     bool InstanceBase::Initialize(const InstanceDescriptor*) {
         return true;
     }
@@ -225,8 +227,15 @@ namespace dawn_native {
         mPlatform = platform;
     }
 
-    dawn_platform::Platform* InstanceBase::GetPlatform() const {
-        return mPlatform;
+    dawn_platform::Platform* InstanceBase::GetPlatform() {
+        if (mPlatform != nullptr) {
+            return mPlatform;
+        }
+
+        if (mDefaultPlatform == nullptr) {
+            mDefaultPlatform = std::make_unique<dawn_platform::Platform>();
+        }
+        return mDefaultPlatform.get();
     }
 
     const XlibXcbFunctions* InstanceBase::GetOrCreateXlibXcbFunctions() {