ui/base/models: Make ComboboxModel::GetItemCount() a constant function.

ui/views/controls/combobox/native_combobox_views_unittest.cc

Index: ui/views/controls/combobox/native_combobox_views_unittest.cc
diff --git a/ui/views/controls/combobox/native_combobox_views_unittest.cc b/ui/views/controls/combobox/native_combobox_views_unittest.cc
index 2474c4ad316d3d872e6e50bc3e8c807cc7e12b97..4143d221ce7f31c7a54b0c1d8171bf43a1722ed2 100644
--- a/ui/views/controls/combobox/native_combobox_views_unittest.cc
+++ b/ui/views/controls/combobox/native_combobox_views_unittest.cc
@@ -50,19 +50,20 @@ class TestCombobox : public views::Combobox {
   DISALLOW_COPY_AND_ASSIGN(TestCombobox);
 };
 
-// A concrete class is needed to test the combobox
+// A concrete class is needed to test the combo box.
 class TestComboboxModel : public ui::ComboboxModel {
  public:
   TestComboboxModel() {}
   virtual ~TestComboboxModel() {}
-  virtual int GetItemCount() {
+
+  // Overridden from ui::ComboboxModel:
+  virtual int GetItemCount() const OVERRIDE {
     return 4;
   }
-  virtual string16 GetItemAt(int index) {
-    EXPECT_GE(index, 0);
-    EXPECT_LT(index, GetItemCount());
+  virtual string16 GetItemAt(int index) OVERRIDE {
     return string16();
   }
+
  private:
   DISALLOW_COPY_AND_ASSIGN(TestComboboxModel);
 };