function normalizeColumnPercentWidths(aColumnPercentWidths) {
		var oColumnIndex = {
			begin: 0,
			mid: 1,
			end: 2
		},
		iSum = aColumnPercentWidths.reduce((a, b) => a + b, 0);
		if (iSum !== 100) {
			// the CSS of the mid column always causes it take the space that remained
			// after sizing the begin and end columns
			aColumnPercentWidths[oColumnIndex.mid] = 100 -
			(aColumnPercentWidths[oColumnIndex.begin] + aColumnPercentWidths[oColumnIndex.end]);
		}
	}
Ensures the sum of all column percent widths is 100.
(Used after converting all the column widths from floats to integers,
to avoid inconsistency of the final sum due to rounding.)

@param {object} aColumnPercentWidths the percent widths of all three columns