Add public API for handling Javascript alerts and prompt.

ios/web/web_state/web_state_delegate_bridge.mm

Index: ios/web/web_state/web_state_delegate_bridge.mm
diff --git a/ios/web/web_state/web_state_delegate_bridge.mm b/ios/web/web_state/web_state_delegate_bridge.mm
index 1e279aa9e41d9cd3d3d808b3f48af67c1fd2cc1a..e0163a0afc47a1817c929642fb05edf5001f705a 100644
--- a/ios/web/web_state/web_state_delegate_bridge.mm
+++ b/ios/web/web_state/web_state_delegate_bridge.mm
@@ -29,4 +29,13 @@ bool WebStateDelegateBridge::HandleContextMenu(
   return NO;
 }
 
+JavaScriptDialogPresenter* WebStateDelegateBridge::GetJavaScriptDialogPresenter(
+    WebState* source) {
+  if ([delegate_ respondsToSelector:
+        @selector(javaScriptDialogPresenterForWebState:)]) {
+    return [delegate_ javaScriptDialogPresenterForWebState:source];
+  }
+  return nullptr;
+}
+
 }  // web