Revert of gpu: Completely remove GpuProcessHostUIShim.

ui/ozone/platform/drm/host/drm_gpu_platform_support_host.cc

Index: ui/ozone/platform/drm/host/drm_gpu_platform_support_host.cc
diff --git a/ui/ozone/platform/drm/host/drm_gpu_platform_support_host.cc b/ui/ozone/platform/drm/host/drm_gpu_platform_support_host.cc
index 01920a85e24de42676fd27f8a98e82cb1a362415..c955cb23b734e3881e6a50d69a8da0c98aae1ce9 100644
--- a/ui/ozone/platform/drm/host/drm_gpu_platform_support_host.cc
+++ b/ui/ozone/platform/drm/host/drm_gpu_platform_support_host.cc
@@ -82,7 +82,7 @@
 }  // namespace
 
 DrmGpuPlatformSupportHost::DrmGpuPlatformSupportHost(DrmCursor* cursor)
-    : cursor_(cursor), weak_ptr_factory_(this) {}
+    : cursor_(cursor) {}
 
 DrmGpuPlatformSupportHost::~DrmGpuPlatformSupportHost() {}
 
@@ -105,23 +105,31 @@
 
 void DrmGpuPlatformSupportHost::OnGpuProcessLaunched(
     int host_id,
-    scoped_refptr<base::SingleThreadTaskRunner> ui_runner,
     scoped_refptr<base::SingleThreadTaskRunner> send_runner,
     const base::Callback<void(IPC::Message*)>& send_callback) {
-  DCHECK(!ui_runner->BelongsToCurrentThread());
-  ui_runner_ = std::move(ui_runner);
   TRACE_EVENT1("drm", "DrmGpuPlatformSupportHost::OnGpuProcessLaunched",
                "host_id", host_id);
   host_id_ = host_id;
-  send_runner_ = std::move(send_runner);
+  send_runner_ = send_runner;
   send_callback_ = send_callback;
 
   for (GpuThreadObserver& observer : gpu_thread_observers_)
     observer.OnGpuProcessLaunched();
-
-  ui_runner_->PostTask(
-      FROM_HERE, base::Bind(&DrmGpuPlatformSupportHost::OnChannelEstablished,
-                            weak_ptr_factory_.GetWeakPtr()));
+}
+
+void DrmGpuPlatformSupportHost::OnChannelEstablished() {
+  TRACE_EVENT0("drm", "DrmGpuPlatformSupportHost::OnChannelEstablished");
+  channel_established_ = true;
+
+  for (GpuThreadObserver& observer : gpu_thread_observers_)
+    observer.OnGpuThreadReady();
+
+  // The cursor is special since it will process input events on the IO thread
+  // and can by-pass the UI thread. This means that we need to special case it
+  // and notify it after all other observers/handlers are notified such that the
+  // (windowing) state on the GPU can be initialized before the cursor is
+  // allowed to IPC messages (which are targeted to a specific window).
+  cursor_->SetDrmCursorProxy(new CursorIPC(send_runner_, send_callback_));
 }
 
 void DrmGpuPlatformSupportHost::OnChannelDestroyed(int host_id) {
@@ -166,21 +174,6 @@
 
 void DrmGpuPlatformSupportHost::UnRegisterHandlerForDrmDisplayHostManager() {
   display_manager_ = nullptr;
-}
-
-void DrmGpuPlatformSupportHost::OnChannelEstablished() {
-  TRACE_EVENT0("drm", "DrmGpuPlatformSupportHost::OnChannelEstablished");
-  channel_established_ = true;
-
-  for (GpuThreadObserver& observer : gpu_thread_observers_)
-    observer.OnGpuThreadReady();
-
-  // The cursor is special since it will process input events on the IO thread
-  // and can by-pass the UI thread. This means that we need to special case it
-  // and notify it after all other observers/handlers are notified such that the
-  // (windowing) state on the GPU can be initialized before the cursor is
-  // allowed to IPC messages (which are targeted to a specific window).
-  cursor_->SetDrmCursorProxy(new CursorIPC(send_runner_, send_callback_));
 }
 
 bool DrmGpuPlatformSupportHost::OnMessageReceivedForDrmDisplayHostManager(