[FIX] point_of_sale: deal with Chrome 50's new height inheritance

Chrome 50 treats percent-height divs inside of auto-height cells as
auto [1]. So from now on it's important that an explicit 'height: 100%' CSS
property is set on parent tds, otherwise you'll end up with elements
with a height of 0.

DO NOT FORWARD-PORT!

[1] https://chromium.googlesource.com/chromium/src/+/8876584335b48c99cf8df552ef4d8efebb131041

addons/point_of_sale/static/src/css/pos.css

@@ -683,6 +683,7 @@
 }
 .point-of-sale .screen .content-cell{
     width:100%;
+    height:100%;
 }
 .point-of-sale .screen .content-cell .content-container{
     height:100%;