Get the view size not window size in ExtJS

If your browser window contain a vertical scrollbar, using Ext.getBody().getWidth() will get the body size with the width of vertical scrollbar.
Example, if the body is 800px. and vertical scroll bar is 50px, so the vew size is only 750px.
If you set width of a component to 800px, you will see a horizontal scrollbar.
So lets use Ext.getBody().getViewSize().width (see docs for detail). It will give us the width of the region you see.