Implement Buffer::MapAsync

MapAsync in dawn_native is fully implemented and only missing
a couple cleanups that can be done once MapRead/WriteAsync are
removed.

MapAsync in dawn_wire is left as a pure shim on top of
MapRead/WriteAsync and will be transitioned to its own commands
in follow-ups.

All MapRead/WriteAsync end2end and validation tests are duplicated
for MapAsync.

Bug: dawn:445

Change-Id: Ib1430b9257149917be19a84f13e0ddd2a8eccc32
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/24260
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>

src/dawn_native/null/DeviceNull.cpp

@@ -312,6 +312,10 @@ namespace dawn_native { namespace null {
         return {};
     }
 
+    MaybeError Buffer::MapAsyncImpl(wgpu::MapMode mode, size_t offset, size_t size) {
+        return {};
+    }
+
     void* Buffer::GetMappedPointerImpl() {
         return mBackingData.get();
     }

src/dawn_native/null/DeviceNull.h

@@ -201,6 +201,7 @@ namespace dawn_native { namespace null {
         // Dawn API
         MaybeError MapReadAsyncImpl() override;
         MaybeError MapWriteAsyncImpl() override;
+        MaybeError MapAsyncImpl(wgpu::MapMode mode, size_t offset, size_t size) override;
         void UnmapImpl() override;
         void DestroyImpl() override;