Implement rudimentary touchscreen support in mus.

components/mus/ws/platform_display.cc

Index: components/mus/ws/platform_display.cc
diff --git a/components/mus/ws/platform_display.cc b/components/mus/ws/platform_display.cc
index d46050bf4f3e516b68fc355b27dbe49135113142..46aa239127d72eb048503a6f184aa89bbebb9601 100644
--- a/components/mus/ws/platform_display.cc
+++ b/components/mus/ws/platform_display.cc
@@ -142,7 +142,8 @@ PlatformDisplay* PlatformDisplay::Create(
 
 DefaultPlatformDisplay::DefaultPlatformDisplay(
     const PlatformDisplayInitParams& init_params)
-    : gpu_state_(init_params.gpu_state),
+    : display_id_(init_params.display_id),
+      gpu_state_(init_params.gpu_state),
       surfaces_state_(init_params.surfaces_state),
       delegate_(nullptr),
       draw_timer_(false, false),
@@ -279,6 +280,10 @@ bool DefaultPlatformDisplay::IsFramePending() const {
   return frame_pending_;
 }
 
+int64_t DefaultPlatformDisplay::GetDisplayId() const {
+  return display_id_;
+}
+
 void DefaultPlatformDisplay::WantToDraw() {
   if (draw_timer_.IsRunning() || frame_pending_)
     return;