Introduce ui.dll / libui.so for the component build.

ui/base/dragdrop/drop_target.h

Index: ui/base/dragdrop/drop_target.h
===================================================================
--- ui/base/dragdrop/drop_target.h	(revision 92815)
+++ ui/base/dragdrop/drop_target.h	(working copy)
@@ -9,6 +9,7 @@
 #include <objidl.h>
 
 #include "base/memory/ref_counted.h"
+#include "ui/ui_api.h"
 
 // Windows interface.
 struct IDropTargetHelper;
@@ -24,7 +25,7 @@
 // before the HWND is deleted too.
 //
 // This class is meant to be used in a STA and is not multithread-safe.
-class DropTarget : public IDropTarget {
+class UI_API DropTarget : public IDropTarget {
  public:
   // Create a new DropTarget associating it with the given HWND.
   explicit DropTarget(HWND hwnd);