[omnibox] Break out SetCaretPos() method and enhance browser test

ios/chrome/browser/ui/omnibox/omnibox_view_ios.mm

Index: ios/chrome/browser/ui/omnibox/omnibox_view_ios.mm
diff --git a/ios/chrome/browser/ui/omnibox/omnibox_view_ios.mm b/ios/chrome/browser/ui/omnibox/omnibox_view_ios.mm
index 41e447d6a6bc53aaf1d55013eee0c4f300899698..fcaff179acace4368e6fc41e21dd5d4ef2463287 100644
--- a/ios/chrome/browser/ui/omnibox/omnibox_view_ios.mm
+++ b/ios/chrome/browser/ui/omnibox/omnibox_view_ios.mm
@@ -242,6 +242,10 @@ void OmniboxViewIOS::SetWindowTextAndCaretPos(const base::string16& text,
     model()->OnChanged();
 }
 
+// TODO(crbug.com/726702): Implement this and have |SetWindowTextAndCaretPos()|
+// call it.
+void OmniboxViewIOS::SetCaretPos(size_t caret_pos) {}
+
 void OmniboxViewIOS::RevertAll() {
   ignore_popup_updates_ = true;
   OmniboxView::RevertAll();