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

chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm

Index: chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm
diff --git a/chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm b/chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm
index d692fc07277bf42f6d1d2a65aefd257f615de183..de3d666812823c1a0c0ee7d313fd739d6750cb50 100644
--- a/chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm
+++ b/chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm
@@ -353,6 +353,11 @@ void OmniboxViewMac::SetWindowTextAndCaretPos(const base::string16& text,
     TextChanged();
 }
 
+void OmniboxViewMac::SetCaretPos(size_t caret_pos) {
+  size_t pos = std::min(caret_pos, GetTextLength());
+  SetSelectedRange(NSMakeRange(pos, 0));
+}
+
 void OmniboxViewMac::EnterKeywordModeForDefaultSearchProvider() {
   // We need to do this first, else |SetSelectedRange()| won't work.
   FocusLocation(true);