message: Remove the last few null=backbuffer hacks
  I missed these in
diff: @@ -186,14 +186,6 @@ namespace backend {
  for (auto location : IterateBitSet(subpassInfo.colorAttachmentsSet)) {
  auto attachmentSlot = subpassInfo.colorAttachments[location];
  auto* tv = currentFramebuffer->GetTextureView(attachmentSlot);
  - // TODO(kainino@chromium.org): the TextureView can only be null
  - // because of the null=backbuffer hack (null representing the
  - // backbuffer). Once that hack is removed (once we have WSI)
  - // this check isn't needed.
  - if (tv == nullptr) {
  - continue;
  - }
  -
  auto* texture = tv->GetTexture();
  if (!EnsureTextureUsage(texture, nxt::TextureUsageBit::OutputAttachment)) {
  builder->HandleError("Unable to ensure texture has OutputAttachment usage");
  @@ -217,14 +209,6 @@ namespace backend {
  for (auto location : IterateBitSet(subpassInfo.colorAttachmentsSet)) {
  auto attachmentSlot = subpassInfo.colorAttachments[location];
  auto* tv = currentFramebuffer->GetTextureView(attachmentSlot);
  - // TODO(kainino@chromium.org): the TextureView can only be null
  - // because of the null=backbuffer hack (null representing the
  - // backbuffer). Once that hack is removed (once we have WSI)
  - // this check isn't needed.
  - if (tv == nullptr) {
  - continue;
  - }
  -
  auto* texture = tv->GetTexture();
  if (texture->IsFrozen()) {
  continue;
  