MacViews: Implement Full Keyboard Access.

ui/views/controls/button/checkbox.cc

Index: ui/views/controls/button/checkbox.cc
diff --git a/ui/views/controls/button/checkbox.cc b/ui/views/controls/button/checkbox.cc
index 2ab281f1943b3cf72f93768858a57efed4760c32..3a96a9ef05dc55599620a4fe6ad317315d3b77ac 100644
--- a/ui/views/controls/button/checkbox.cc
+++ b/ui/views/controls/button/checkbox.cc
@@ -28,7 +28,13 @@ Checkbox::Checkbox(const base::string16& label)
   // Inset the trailing side by a couple pixels for the focus border.
   button_border->set_insets(gfx::Insets(0, 0, 0, 2));
   SetBorder(std::move(button_border));
+
+#if !defined(OS_MACOSX)

[tapted, 2016/02/12 02:56:44]

It's usually better to avoid negatives in conditions if you can neatly. I think
the SetFocusable(false) should be redundant too, so this could be

#if defined(OS_MACOSX)
SetAccessibilityFocusable(true);
#else
SetFocusable(false);
#endif

But also button.cc already has `SetAccessibilityFocusable(true);` so perhaps we
just need 

#if !defined(OS_MACOSX)
SetFocusable(false);
#endif


But!

I nicer solution might be to do something like

PlatformStyle::SetButtonStyleFocus(View* view);

replace all the current SetFocusable(true) calls with
PlatformStyle::SetButtonStyleFocus(this) and provide platform implementations in
platform_style.cc and platform_style_mac.cc. I think this is the nicest option.

[karandeepb, 2016/02/22 07:15:37]

I have added a PlatformStyle::SetControlStyleFocus instead, since all views
which represent controls(except textfields and lists) should have this focus
setting. 

   SetFocusable(true);
+#else
+  SetFocusable(false);
+  SetAccessibilityFocusable(true);
+#endif
 
   ui::ResourceBundle& rb = ui::ResourceBundle::GetSharedInstance();