Only use CompositorNativeWindow11 when the window is non-null.

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/1333967
Reviewed-by: James Clarke <james.clarke@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>

src/libANGLE/renderer/d3d/d3d11/Renderer11.cpp

@@ -1272,7 +1272,7 @@ NativeWindowD3D *Renderer11::createNativeWindow(EGLNativeWindowType window,
                                                 const egl::Config *config,
                                                 const egl::AttributeMap &attribs) const
 {
-    auto useWinUiComp = !NativeWindow11Win32::IsValidNativeWindow(window);
+    auto useWinUiComp = window != nullptr && !NativeWindow11Win32::IsValidNativeWindow(window);
 
     if (useWinUiComp)
     {