attempted fix for layoutpanel child widget width issues

Recent versions of Chrome and Chromium no longer allow
child widgets to completely fill their parent containers
event if using absolute layout and setting "right: 0px;".
These is reminiscent of the issues we had with FireFox
(GWT issue 4348).  My attempted work around is very
similar: set a width property on the child widget to attempt
to make it fill its parent.  Setting the boxSizing
property to "border-box" also seems to be necessary.