[Autofill Sync] Adding new row type

This CL introduces the frontend changes for an upcoming new row type in
the Autofill dropdown footer. Specifically, it:
* Adds a new frontend ID type.
* Checks against this frontend ID to the child view creation loop.
* Extracts the logic for choosing the layout type into a virtual method,
  replacing the anonymous helper and the optional override parameter
  used by the Footer rows to modify the layout type.
* Uses this new method to set the layout type in this new row type.
* Adds a frontend_id() convenience method to ItemView.

For more details see (Googlers only):
https://docs.google.com/document/d/18oV-_0QfoWM7rgp-suzDn4WspFNHRPKxI-4epHDQZIg/edit?usp=sharing

Bug: 905052
Change-Id: I370b3dc86a4fd7abc7dffa606acd2bd68213c4ba
Reviewed-on: https://chromium-review.googlesource.com/c/1334415
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609730}

chrome/browser/ui/autofill/autofill_popup_layout_model.cc

@@ -181,6 +181,7 @@ const gfx::FontList& AutofillPopupLayoutModel::GetValueFontListForRow(
     case POPUP_ITEM_ID_ALL_SAVED_PASSWORDS_ENTRY:
     case POPUP_ITEM_ID_GENERATE_PASSWORD_ENTRY:
     case POPUP_ITEM_ID_GOOGLE_PAY_BRANDING:
+    case POPUP_ITEM_ID_SHOW_ACCOUNT_CARDS:
       return normal_font_list_;
     case POPUP_ITEM_ID_AUTOCOMPLETE_ENTRY:
     case POPUP_ITEM_ID_DATALIST_ENTRY:

components/autofill/core/browser/popup_item_ids.h

@@ -27,6 +27,7 @@ enum PopupItemId {
   POPUP_ITEM_ID_ALL_SAVED_PASSWORDS_ENTRY = -13,
   POPUP_ITEM_ID_GOOGLE_PAY_BRANDING = -14,
   POPUP_ITEM_ID_GENERATE_PASSWORD_ENTRY = -15,
+  POPUP_ITEM_ID_SHOW_ACCOUNT_CARDS = -16,
 };
 
 }  // namespace autofill