[Autofill iOS] Add guards when accessing iframe.

These methods can be calle asynchronously.
It is necessary to check that the web_frame was not destroyed before
processing it.

Bug: 942427
Change-Id: Icc46dae9193c4fd9ef9612ce47239bd50eef33d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538194
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644779}

components/autofill/ios/browser/autofill_driver_ios.mm

@@ -90,6 +90,9 @@ void AutofillDriverIOS::SendFormDataToRenderer(
     RendererFormDataAction action,
     const FormData& data) {
   web::WebFrame* web_frame = web::GetWebFrameWithId(web_state_, web_frame_id_);
+  if (!web_frame) {
+    return;
+  }
   [bridge_ fillFormData:data inFrame:web_frame];
 }
 
@@ -101,6 +104,9 @@ void AutofillDriverIOS::PropagateAutofillPredictions(
 void AutofillDriverIOS::SendAutofillTypePredictionsToRenderer(
     const std::vector<FormStructure*>& forms) {
   web::WebFrame* web_frame = web::GetWebFrameWithId(web_state_, web_frame_id_);
+  if (!web_frame) {
+    return;
+  }
   [bridge_ fillFormDataPredictions:FormStructure::GetFieldTypePredictions(forms)
                            inFrame:web_frame];
 }