Made the horizontal border between chat and presentation areas one pixel wide on mobile.

bigbluebutton-html5/app/client/stylesheets/chat.less

@@ -74,6 +74,9 @@
     border-top-left-radius: 0px;
     overflow: hidden;
   }
+  @media @mobile-portrait-with-keyboard, @mobile-portrait {
+    border-top: none;
+  }
   order: 3;
 }