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

Review URL: https://codereview.chromium.org/186123004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255398 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255398