Remove incorrect DCHECK

As per
https://cs.chromium.org/chromium/src/components/viz/service/display/direct_renderer.cc?rcl=a70a3dd3c62b530b00367ba545383a91e51fe91e&l=124,
we do allow empty sub_buffer_rect when partial swap is enabled.
This codepath was added in https://codereview.chromium.org/1489153002
to support updating overlays without changing the framebuffer contents.

Bug: 963861
Change-Id: Ia18e5b34a8e48dfd448e7f8dc2bd02e07805c078
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1658770
Commit-Queue: Jonathan Backer <backer@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668975}

components/viz/service/display_embedder/skia_output_surface_impl_on_gpu.cc

@@ -716,8 +716,11 @@ void SkiaOutputSurfaceImplOnGpu::SwapBuffers(OutputSurfaceFrame frame) {
 
   DCHECK(output_device_);
   gfx::SwapResponse response;
-  if (capabilities().supports_post_sub_buffer && frame.sub_buffer_rect) {
-    DCHECK(!frame.sub_buffer_rect->IsEmpty());
+  if (frame.sub_buffer_rect && frame.sub_buffer_rect->IsEmpty()) {
+    // TODO(https://crbug.com/898680): Maybe do something for overlays here.
+    // This codepath was added in https://codereview.chromium.org/1489153002
+    // to support updating overlays without changing the framebuffer contents.
+  } else if (capabilities().supports_post_sub_buffer && frame.sub_buffer_rect) {
     if (!capabilities().flipped_output_surface)
       frame.sub_buffer_rect->set_y(size_.height() - frame.sub_buffer_rect->y() -
                                    frame.sub_buffer_rect->height());