dawn_native: Add RequestDevice to the Adapter

Adds a basic RequestDevice method to the adapter, only in
dawn_native. We will revisit this when we implement adapters
in dawn_wire.

RequestDevice allows us to have tests of the limit bounds
because it receives a callback which can return status codes
and error messages.

Bug: dawn:685
Change-Id: I7a68922b078c6a436f49a16346cb41fb9df9cfee
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/63982
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/include/dawn_native/DawnNative.h

@@ -120,6 +120,10 @@ namespace dawn_native {
         // On an error, nullptr is returned.
         WGPUDevice CreateDevice(const DeviceDescriptor* deviceDescriptor = nullptr);
 
+        void RequestDevice(const DeviceDescriptor* descriptor,
+                           WGPURequestDeviceCallback callback,
+                           void* userdata);
+
         // Reset the backend device object for testing purposes.
         void ResetInternalDeviceForTesting();