[ios] Added API to enable/disable web usage for WebState.

ios/web/public/web_state/web_state.h

Index: ios/web/public/web_state/web_state.h
diff --git a/ios/web/public/web_state/web_state.h b/ios/web/public/web_state/web_state.h
index 0a7904930dd7f216fa1f45a18ca1a927410dc03f..d9dfe653e0a1beeb14bf928a6acf244a663a26b6 100644
--- a/ios/web/public/web_state/web_state.h
+++ b/ios/web/public/web_state/web_state.h
@@ -90,6 +90,11 @@ class WebState : public base::SupportsUserData {
   virtual WebStateDelegate* GetDelegate() = 0;
   virtual void SetDelegate(WebStateDelegate* delegate) = 0;
 
+  // Whether or not a web view is allowed to exist in this WebState. Defaults
+  // to false; this should be enabled before attempting to access the view.
+  virtual bool IsWebUsageEnabled() const = 0;
+  virtual void SetWebUsageEnabled(bool enabled) = 0;
+
   // The view containing the contents of the current web page. If the view has
   // been purged due to low memory, this will recreate it. It is up to the
   // caller to size the view.