Revert 255385 "aura: Remove client::UserActionClient."

Revert 255385 "aura: Remove client::UserActionClient."

> aura: Remove client::UserActionClient.
> 
> UserActionClient is used to navigate back/forward when the back/forward on a
> supported mouse-device is clicked on X11. This can instead be achieved by
> installing an event-handler on the browser window.
> 
> This does change the behaviour a little on ChromeOS: clicking these navigation
> buttons when the cursor is not on top of the browser window will not navigate
> after this change. I have confirmed with UX that this is a desirable change.
> 
> BUG=319636
> R=sky@chromium.org
> 
> Review URL: https://codereview.chromium.org/183853037

TBR=sadrul@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=255398

[Ben Goodger (Google), 2014-03-06 18:08:01]

Committed patchset #1 manually as r255398 (tree was closed).