[ios] Added WebState::GetLoadingProgress web// API.

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 c8f310927adbaf94c75767293dc74f58a5c74d6c..5b1bb62096e4455c31730c3aa9dafc67426ef2da 100644
--- a/ios/web/public/web_state/web_state.h
+++ b/ios/web/public/web_state/web_state.h
@@ -151,6 +151,10 @@ class WebState : public base::SupportsUserData {
   // Returns true if the current page is loading.
   virtual bool IsLoading() const = 0;
 
+  // The fraction of the page load that has completed as a number between 0.0
+  // (nothing loaded) and 1.0 (fully loaded).
+  virtual double GetLoadingProgress() const = 0;
+
   // Whether this instance is in the process of being destroyed.
   virtual bool IsBeingDestroyed() const = 0;