Fix DCHECK on form activity post page unload with frame message disabled

Without frame messaging, messages can be receive after the frame is
deleted.
The current code DCHECKs because the frameID is empty in that case.

Change-Id: I008803da50f166bfefe29159addaac2a69f15ec1
Reviewed-on: https://chromium-review.googlesource.com/c/1256804
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596154}

components/autofill/ios/browser/autofill_agent.mm

@@ -371,8 +371,7 @@ autofillManagerFromWebState:(web::WebState*)webState
                       webState:(web::WebState*)webState
              completionHandler:(SuggestionsAvailableCompletion)completion {
   web::WebFrame* frame =
-      web::WebFramesManager::FromWebState(webState)->GetFrameWithId(
-          base::SysNSStringToUTF8(frameID));
+      GetWebFrameWithId(webState, base::SysNSStringToUTF8(frameID));
   autofill::AutofillManager* autofillManager =
       [self autofillManagerFromWebState:webState webFrame:frame];
   if (!autofillManager)
@@ -574,6 +573,7 @@ autofillManagerFromWebState:(web::WebState*)webState
 - (void)processPage:(web::WebState*)webState {
   web::WebFramesManager* framesManager =
       web::WebFramesManager::FromWebState(webState);
+  DCHECK(framesManager);
   if (!autofill::switches::IsAutofillIFrameMessagingEnabled()) {
     [self processFrame:framesManager->GetMainWebFrame() inWebState:webState];
   } else {