fix incorrect mac drop coordinate

The OnDragEnd coordinates was incorrect at mac,
because the api we are using:
draggedImage:endedAt:operation
(https://developer.apple.com/documentation/appkit/
nsdraggingsource/1806949-draggedimage?language=objc)
The endedAt location is the image’s origin in screen
coordinate, but we used it as dragend coordinate.

In this patch, I added a offset similar to startDrag
to correct the coordinates.

Bug: 762701
Change-Id: Ie92e84998d076c58f099215a548ab1934c973958
Reviewed-on: https://chromium-review.googlesource.com/655977Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510234}

content/browser/web_contents/web_drag_source_mac.mm

@@ -244,6 +244,12 @@ void PromiseWriterHelper(const DropData& drop_data,
 
   contents_->SystemDragEnded(dragStartRWH_.get());
 
+  if (dragImage_) {
+    screenPoint.x += imageOffset_.x;
+    // Deal with Cocoa's flipped coordinate system.
+    screenPoint.y += [dragImage_.get() size].height - imageOffset_.y;
+  }
+
   // Convert |screenPoint| to view coordinates and flip it.
   NSPoint localPoint = NSZeroPoint;
   if ([contentsView_ window])