Clean up text_example.* some.

Remove DISALLOW_COPY, use constexpr more, move constants as close to
first use as possible, etc.

This was originally motivated by wanting to change the
Combobox::set_callback() call to set_closure(); that's present here, but
I couldn't help cleaning up at least some of the rest of this too.

Bug: none
Change-Id: I5539b0b9d11a534d059107f57dd91d0f65b7a0a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2420778
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809159}

ui/views/examples/text_example.h

@@ -32,13 +32,19 @@ class VIEWS_EXAMPLES_EXPORT TextExample : public ExampleBase,
   // Creates and adds a check box to the layout.
   Checkbox* AddCheckbox(GridLayout* layout, const char* name);
 
-  // Creates and adds a combobox to the layout.
+  // Creates and adds a combobox to the layout. Sets |this|' instance of
+  // |combobox_callback| as the callback for the created combobox.
   Combobox* AddCombobox(GridLayout* layout,
                         const char* name,
                         const char* const* strings,
-                        int count);
-
-  void OnPerformAction(Combobox* combobox);
+                        int count,
+                        void (TextExample::*combobox_callback)());
+
+  void AlignComboboxChanged();
+  void TextComboboxChanged();
+  void ElideComboboxChanged();
+  void PrefixComboboxChanged();
+  void WeightComboboxChanged();
 
   // ButtonListener:
   void ButtonPressed(Button* button, const ui::Event& event) override;