Allow OSExchangeData::Provider to be overridden at run time.

ui/views/cocoa/drag_drop_client_mac.mm

Index: ui/views/cocoa/drag_drop_client_mac.mm
diff --git a/ui/views/cocoa/drag_drop_client_mac.mm b/ui/views/cocoa/drag_drop_client_mac.mm
index ed85acbfe9eff246b8cbfd2aef8c763214ace781..83b70ed948f4c76c3bf142c87b179bbe698fc525 100644
--- a/ui/views/cocoa/drag_drop_client_mac.mm
+++ b/ui/views/cocoa/drag_drop_client_mac.mm
@@ -25,7 +25,8 @@
 
 - (id)initWithData:(const ui::OSExchangeData&)data {
   if ((self = [super init])) {
-    data_.reset(new OSExchangeData(data.provider().Clone()));
+    data_.reset(new OSExchangeData(
+        std::unique_ptr<OSExchangeData::Provider>(data.provider().Clone())));
   }
   return self;
 }