app_shell: Fix PrefsTabHelper dependency of ExtensionHost

extensions/browser/extensions_browser_client.h

Index: extensions/browser/extensions_browser_client.h
diff --git a/extensions/browser/extensions_browser_client.h b/extensions/browser/extensions_browser_client.h
index c93b629995a3519022aef34292b88b4592aa69d3..52e3b3629c4bf98db5e5ebd8e3bea158f4f5ff28 100644
--- a/extensions/browser/extensions_browser_client.h
+++ b/extensions/browser/extensions_browser_client.h
@@ -13,6 +13,7 @@ class PrefService;
 namespace content {
 class BrowserContext;
 class JavaScriptDialogManager;
+class WebContents;
 }
 
 namespace extensions {
@@ -71,6 +72,10 @@ class ExtensionsBrowserClient {
   virtual bool IsBackgroundPageAllowed(
       content::BrowserContext* context) const = 0;
 
+  // Called after the hosting |web_contents| for an extension is created. The
+  // implementation may wish to add preference observers to |web_contents|.
+  virtual void OnExtensionHostCreated(content::WebContents* web_contents) = 0;
+
   // Returns true if the client version has updated since the last run. Called
   // once each time the extensions system is loaded per browser_context. The
   // implementation may wish to use the BrowserContext to record the current