Fix deadlock between UI thread and drag and drop thread on Windows.

ui/base/dragdrop/os_exchange_data.h

Index: ui/base/dragdrop/os_exchange_data.h
diff --git a/ui/base/dragdrop/os_exchange_data.h b/ui/base/dragdrop/os_exchange_data.h
index f55dd2d62ed633cf3983c9b2184a3e785ae4be32..37127c76ba04deaedc1d8076ce92888242a3130d 100644
--- a/ui/base/dragdrop/os_exchange_data.h
+++ b/ui/base/dragdrop/os_exchange_data.h
@@ -135,6 +135,7 @@ class UI_EXPORT OSExchangeData {
                                  std::string* file_contents) const = 0;
     virtual bool HasFileContents() const = 0;
     virtual void SetDownloadFileInfo(const DownloadFileInfo& download) = 0;
+    virtual void SetInDragLoop(bool in_drag_loop) = 0;
 #endif
 
 #if defined(OS_WIN) || defined(USE_AURA)
@@ -227,6 +228,8 @@ class UI_EXPORT OSExchangeData {
 
   // Adds a download file with full path (CF_HDROP).
   void SetDownloadFileInfo(const DownloadFileInfo& download);
+
+  void SetInDragLoop(bool in_drag_loop);
 #endif
 
 #if defined(OS_WIN) || defined(USE_AURA)