Make close button visible when focusing inline actions.

Make close button visible when focusing inline actions.

When inline action button is focused by keyboard, the close button
becomes invisible. That's why we can't focus the close button by hitting
TAB key repeatedly.
This CL fixes it by making control buttons (including close button)
visible when the notification or its descendant views have focus.

BUG=731074
TEST=manually tested

[fukino, 2017-06-08 12:51:33]

fukino@chromium.org changed reviewers:
+ yoshiki@chromium.org

[fukino, 2017-06-08 12:51:33]

yoshiki@, could you take a look?

[yoshiki, 2017-06-09 08:32:35]

At least on ARC notification, the close button shouldn't be visible when the
focus is on notification except for buttons, because some elements on
notification might be placed under the buttons.

Should we keep the consistency with ARC notifications?

[fukino, 2017-06-09 08:50:34]

On 2017/06/09 08:32:35, yoshiki wrote:
> At least on ARC notification, the close button shouldn't be visible when the
> focus is on notification except for buttons, because some elements on
> notification might be placed under the buttons.
> 
> Should we keep the consistency with ARC notifications?

I got it.
When the inline actions are focused, the close button should be invisible but
need to be focused.
Using alpha instead if SetVisible() might interfere the clickable area on the
header, so I'll look for another approach.