Stop calling "focus" if a target iframe already has focus.

In IE10, if "focus" is applied to an iframe window, it becomes impossible to get the right range.


# Steps to reproduce this bug
1. Input `<p>test 123</p>`.
2. Select  from very top of the document to "p" element.
3. Click "Bold" button.

# Expected
* `<p><bold>test 123</bold></p>`

# Result
* `<p></p><p>test 123</p>`

# Screencast
http://www.screencast.com/t/lUYp228dg

# Comment
Although this bug seldom occurs when the default content_css is used, 
but when the custom content_css that has a large margin is being used, it occurs frequently.