https://cs.chromium.org/chromium/src/chrome/browser/ui/cocoa/share_menu_controller.mm?type=cs&g=0&l=161


  TabWindowController* tabWindowController =
      TabWindowControllerForWindow(windowForShare_);
  NSView* contentsView = [tabWindowController tabContentArea];
  NSRect rectInWindow =
      [[contentsView superview] convertRect:[contentsView frame] toView:nil];
  rectForShare_ = [windowForShare_ convertRectToScreen:rectInWindow];

  gfx::Image image;
  gfx::Rect rect = gfx::Rect(NSRectToCGRect([contentsView bounds]));
  if (ui::GrabViewSnapshot(contentsView, rect, &image)) {
    snapshotForShare_.reset(image.CopyNSImage());
  }



But:

// Returns the TabWindowController backing a Cocoa browser window. Always
// returns nil if |window| is a views browser window.
TabWindowController* TabWindowControllerForWindow(NSWindow* window);


So the code probably doesn't do what we want.