Make using WebContentsUserData simpler.

chrome/browser/extensions/api/web_navigation/web_navigation_api.cc

Index: chrome/browser/extensions/api/web_navigation/web_navigation_api.cc
diff --git a/chrome/browser/extensions/api/web_navigation/web_navigation_api.cc b/chrome/browser/extensions/api/web_navigation/web_navigation_api.cc
index ed19fcf39b877212f1a13fb2977bddd5afbd556e..1b1e2f10a36dc6a1ac67ea7a826ff25576df3c22 100644
--- a/chrome/browser/extensions/api/web_navigation/web_navigation_api.cc
+++ b/chrome/browser/extensions/api/web_navigation/web_navigation_api.cc
@@ -31,6 +31,8 @@
 namespace GetFrame = extensions::api::web_navigation::GetFrame;
 namespace GetAllFrames = extensions::api::web_navigation::GetAllFrames;
 
+DEFINE_WEB_CONTENTS_USER_DATA_KEY(extensions::WebNavigationTabObserver)
+
 namespace extensions {
 
 namespace helpers = web_navigation_api_helpers;
@@ -253,8 +255,6 @@ void WebNavigationEventRouter::TabDestroyed(content::WebContents* tab) {
 
 // WebNavigationTabObserver ------------------------------------------
 
-int WebNavigationTabObserver::kUserDataKey;
-
 WebNavigationTabObserver::WebNavigationTabObserver(
     content::WebContents* web_contents)
     : WebContentsObserver(web_contents),