[Mac] Fix cursor when dropping to desktop

Allow mouse cursor to be a green plus when dropping items to desktop

Bug: 905238
Change-Id: I90a6f7156e681271f577f1f604f1d75ada4beba2
Reviewed-on: https://chromium-review.googlesource.com/c/1334994Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609410}

content/browser/web_contents/web_contents_view_mac.mm

@@ -172,7 +172,8 @@ void WebContentsViewMac::StartDragging(
   // The drag invokes a nested event loop, arrange to continue
   // processing events.
   base::MessageLoopCurrent::ScopedNestableTaskAllower allow;
-  NSDragOperation mask = static_cast<NSDragOperation>(allowed_operations);
+  NSDragOperation mask = static_cast<NSDragOperation>(allowed_operations) &
+                         ~NSDragOperationGeneric;
   NSPoint offset = NSPointFromCGPoint(
       gfx::PointAtOffsetFromOrigin(image_offset).ToCGPoint());
   [cocoa_view_ startDragWithDropData:drop_data