Implement rudimentary touchscreen support in mus.

components/mus/ws/platform_display.h

Index: components/mus/ws/platform_display.h
diff --git a/components/mus/ws/platform_display.h b/components/mus/ws/platform_display.h
index e733e252e59f0ec3f02b3645e57b8b440240d7d5..9b19220f23469c7adea1c198ef0fc0ade5fa6b9f 100644
--- a/components/mus/ws/platform_display.h
+++ b/components/mus/ws/platform_display.h
@@ -98,6 +98,8 @@ class PlatformDisplay {
   virtual void RequestCopyOfOutput(
       std::unique_ptr<cc::CopyOutputRequest> output_request) = 0;
 
+  virtual int64_t GetDisplayId() const = 0;
+
   // Overrides factory for testing. Default (NULL) value indicates regular
   // (non-test) environment.
   static void set_factory_for_testing(PlatformDisplayFactory* factory) {
@@ -133,6 +135,7 @@ class DefaultPlatformDisplay : public PlatformDisplay,
   bool IsFramePending() const override;
   void RequestCopyOfOutput(
       std::unique_ptr<cc::CopyOutputRequest> output_request) override;
+  int64_t GetDisplayId() const override;
 
  private:
   void WantToDraw();
@@ -162,6 +165,8 @@ class DefaultPlatformDisplay : public PlatformDisplay,
   void OnAcceleratedWidgetDestroyed() override;
   void OnActivationChanged(bool active) override;
 
+  int64_t display_id_;
+
   scoped_refptr<GpuState> gpu_state_;
   scoped_refptr<SurfacesState> surfaces_state_;
   PlatformDisplayDelegate* delegate_;