Plumb GpuSwapBuffers completion from Mus GPU thread to WS thread

components/mus/surfaces/surfaces_context_provider.cc

Index: components/mus/surfaces/surfaces_context_provider.cc
diff --git a/components/mus/surfaces/surfaces_context_provider.cc b/components/mus/surfaces/surfaces_context_provider.cc
index 6995806b6f207026ccd61c8b3ca944150e68e4bb..434ad60d3a3c1a754c7e6cc5a65fa3358ae71a9b 100644
--- a/components/mus/surfaces/surfaces_context_provider.cc
+++ b/components/mus/surfaces/surfaces_context_provider.cc
@@ -114,4 +114,15 @@ void SurfacesContextProvider::DidLoseContext() {
   lost_context_callback_.Run();
 }
 
+void SurfacesContextProvider::GpuCompletedSwapBuffers(gfx::SwapResult result) {
+  if (!swap_buffers_completion_callback_.is_null()) {
+    swap_buffers_completion_callback_.Run(result);
+  }
+}
+
+void SurfacesContextProvider::SetSwapBuffersCompletionCallback(
+    gfx::GLSurface::SwapCompletionCallback callback) {
+  swap_buffers_completion_callback_ = callback;
+}
+
 }  // namespace mus