SurfaceD3D always instantiates a NativeWindow object even for pbuffers.
A null window pointer now creates a CompositorNativeWindow11 instead of
a NativeWindow11Win32 which instantiates an RoHelper in the constructor.
This is expensive for some use cases that create a pbuffer surface every
frame.

BUG=angleproject:2956

Change-Id: If43930d4960c2510b6b96291aab971e3f3abf665
Reviewed-on: https://chromium-review.googlesource.com/c/1333967Reviewed-by: James Clarke <james.clarke@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>