function getColsFromBreakpoint(breakpoint
/*: Breakpoint*/
, cols
/*: Breakpoints*/
)
/*: number*/
{
  if (!cols[breakpoint]) {
    throw new Error("ResponsiveGridLayout: `cols` entry for breakpoint " + breakpoint + " is missing!");
  }

  return cols[breakpoint];
}
Given a breakpoint, get the # of cols set for it.
@param  {String} breakpoint Breakpoint name.
@param  {Object} cols       Map of breakpoints to cols.
@return {Number}            Number of cols.