Switch Extensions::TabHelper to use WebContents, WebContentsUserData.

chrome/browser/ui/cocoa/tabpose_window.mm

Index: chrome/browser/ui/cocoa/tabpose_window.mm
diff --git a/chrome/browser/ui/cocoa/tabpose_window.mm b/chrome/browser/ui/cocoa/tabpose_window.mm
index 056519ab0c0fcd8f676c432daf3b86c62a397b17..7bd20f11229d5b381f5da79baec34b46343a5699 100644
--- a/chrome/browser/ui/cocoa/tabpose_window.mm
+++ b/chrome/browser/ui/cocoa/tabpose_window.mm
@@ -457,8 +457,10 @@ NSRect Tile::GetFaviconStartRectRelativeTo(const Tile& tile) const {
 }
 
 NSImage* Tile::favicon() const {
-  if (contents_->extension_tab_helper()->is_app()) {
-    SkBitmap* bitmap = contents_->extension_tab_helper()->GetExtensionAppIcon();
+  extensions::TabHelper* extensions_tab_helper =
+      extensions::TabHelper::FromWebContents(contents_->web_contents());
+  if (extensions_tab_helper->is_app()) {
+    SkBitmap* bitmap = extensions_tab_helper->GetExtensionAppIcon();
     if (bitmap)
       return gfx::SkBitmapToNSImage(*bitmap);
   }