Select <style> more strictly

[Westbrook]

Select only direct children because you need to be able to user "insertBefore" later

Fixes #4975

ToDo:

• confirm x-browser (:scope might not be the answer because of that)
• add tests

[Westbrook]

Reading up on the very clever and interesting :scope > style selector, it looks like there may be some issue with x-browser support, I need to confirm on a system with IE, but in that case would something like the following be overkill:

function processElementStyles(klass, template, is, baseURI) {
      const styles = Polymer.StyleGather.stylesFromModuleImports(is).concat(
        Polymer.StyleGather.stylesFromTemplate(template));
      let templateStyles = template.content.querySelectorAll('style');
      let lastDirectChildStyleIndex = templateStyles.length-1
      let lastDirectChildStyle = templateStyles[lastDirectChildStyleIndex];
      // a direct child of the tempalte is needed for application of
      // "included" styles below via `insertBefore`
      while (!lastDirectChildStyle.parentNode.isSameNode(template.content)) {
        lastDirectChildStyleIndex--;
        if (lastDirectChildStyleIndex < 0) {
          // if no direct child style elements, choose the first element child
          lastDirectChildStyle = template.content.firstElementChild;
        } else {
          lastDirectChildStyle = templateStyles[lastDirectChildStyleIndex];
        }
      }
      // ensure all gathered styles are actually in this template.
      for (let i=0; i < styles.length; i++) {
        let s = styles[i];
        // if the style is not in this template, it's been "included" and
        // we put a clone of it in the template.
        if (s.getRootNode() != template.content) {
          s = s.cloneNode(true);
          template.content.insertBefore(s, lastDirectChildStyle);
        }
        s.textContent = klass._processStyleText(s.textContent, baseURI);
      }
      if (window.ShadyCSS) {
        window.ShadyCSS.prepareTemplate(template, is);
      }
    }

Reading it back here, should it always be template.content.firstElementChild, which allows to a removal of all the back checking?