function getLineHeight(el, style) {
    var lineHeightStr = style.lineHeight;

    if (lineHeightStr) {
      if (lineHeightStr.indexOf('px') < 0) {
        throw Error('The ellipsis container ' + elementName(el) + ' must have line-height set using px unit, found: ' + lineHeightStr);
      }

      var lineHeight = parseInt(lineHeightStr, 10);
      if (lineHeight) {
        return lineHeight;
      }
    }
    throw Error('The ellipsis container ' + elementName(el) + ' must have line-height set on it, found: ' + lineHeightStr);
  }
Gets the line height
from the style declaration.

@param  {CSSStyleDeclaration} style
@return {Number|null}
@api private