Also use ExternalImageDescriptor in D3D12 and Metal Backends

Bug: chromium:1036080
Change-Id: I358b0441c168ca075b09ab028c2f6d9df2a44d59
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/16240
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Natasha Lee <natlee@microsoft.com>

src/dawn_native/DawnNative.cpp

@@ -171,4 +171,8 @@ namespace dawn_native {
         return GetProcMapNamesForTestingInternal();
     }
 
+    ExternalImageDescriptor::ExternalImageDescriptor(ExternalImageDescriptorType type)
+        : type(type) {
+    }
+
 }  // namespace dawn_native