Add Vector2d classes that represent offsets, instead of using Point.

content/browser/web_contents/web_contents_view_mac.mm

Index: content/browser/web_contents/web_contents_view_mac.mm
diff --git a/content/browser/web_contents/web_contents_view_mac.mm b/content/browser/web_contents/web_contents_view_mac.mm
index b619774ef0536fa303e7e1f00ceff91fa4a9d104..342a0517f0da599442bfc95ad15845324284aaaa 100644
--- a/content/browser/web_contents/web_contents_view_mac.mm
+++ b/content/browser/web_contents/web_contents_view_mac.mm
@@ -174,7 +174,7 @@ void WebContentsViewMac::StartDragging(
     const WebDropData& drop_data,
     WebDragOperationsMask allowed_operations,
     const gfx::ImageSkia& image,
-    const gfx::Point& image_offset) {
+    const gfx::Vector2d& image_offset) {
   // By allowing nested tasks, the code below also allows Close(),
   // which would deallocate |this|.  The same problem can occur while
   // processing -sendEvent:, so Close() is deferred in that case.
@@ -186,7 +186,8 @@ void WebContentsViewMac::StartDragging(
   // processing events.
   MessageLoop::ScopedNestableTaskAllower allow(MessageLoop::current());
   NSDragOperation mask = static_cast<NSDragOperation>(allowed_operations);
-  NSPoint offset = NSPointFromCGPoint(image_offset.ToCGPoint());
+  NSPoint offset = NSPointFromCGPoint(
+      gfx::PointAtOffsetFromOrigin(image_offset).ToCGPoint());
   [cocoa_view_ startDragWithDropData:drop_data
                    dragOperationMask:mask
                                image:gfx::NSImageFromImageSkia(image)