[AF][IOS] Scan page for form changes when a field is focused

This CL fixes a regression caused by crrev.com/c/1012979. Previously,
AutofillAgent would scan the whole page on form activities due to the
possiblity of Autofill having been enabled after page load.
crrev.com/c/1012979 replaced that logic in favor of observing the Pref
directly. This has a side effect where previously hidden fields can go
unnoticed by AutofillManager. This CL fixes that regression by scanning the
page when a field is focused. All fields receive focuse before their value
is set by Autofill, therefore, AutofillManager can get notified of
previously hidden fields.

Bug: 855696
Change-Id: I175bf90e12d1ca356b746e6347943a295063c288
Reviewed-on: https://chromium-review.googlesource.com/1112493Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570713}

components/autofill/ios/browser/autofill_agent.mm

@@ -634,10 +634,10 @@ void GetFormAndField(autofill::FormData* form,
   web::URLVerificationTrustLevel trustLevel;
   const GURL pageURL(webState->GetCurrentURL(&trustLevel));
 
-  // If the event is a form_changed, then the event concerns the whole page and
-  // not a particular form. The whole page need to be reparsed to find the new
-  // forms.
-  if (params.type == "form_changed") {
+  // Scan the page to find the latest version of the forms for their new,
+  // changed or previously hidden fields, when we get notified of form changes
+  // or a field is focused.
+  if (params.type == "form_changed" || params.type == "focus") {
     [self scanFormsInPage:webState pageURL:pageURL];
     return;
   }