[Autofill][iOS]Only scan main frame if frame messaging is disabled.

Tbr: mahmadi@chromium.org
Bug: 881364
Change-Id: I763f982645697fb35a2691e96159904154030d08
Reviewed-on: https://chromium-review.googlesource.com/1238548Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593506}

components/autofill/ios/browser/autofill_agent.mm

@@ -574,9 +574,13 @@ autofillManagerFromWebState:(web::WebState*)webState
 - (void)processPage:(web::WebState*)webState {
   web::WebFramesManager* framesManager =
       web::WebFramesManager::FromWebState(webState);
+  if (!autofill::switches::IsAutofillIFrameMessagingEnabled()) {
+    [self processFrame:framesManager->GetMainWebFrame() inWebState:webState];
+  } else {
     for (auto* frame : framesManager->GetAllWebFrames()) {
       [self processFrame:frame inWebState:webState];
     }
+  }
 }
 
 - (void)processFrame:(web::WebFrame*)frame inWebState:(web::WebState*)webState {