Remove template_process_html() [#1843704]

Problem/Motivation

We are eliminating the process layer from Drupal 8, since the reasons for it's existence can now be solved in different ways. One of the functions on the chopping block is template_process_html().

function template_process_html(&$variables) {
  drupal_add_library('system', 'html5shiv', TRUE);
  // Render page_top and page_bottom into top level variables.
  $variables['page_top'] = drupal_render($variables['page']['page_top']);
  $variables['page_bottom'] = drupal_render($variables['page']['page_bottom']);
  // Place the rendered HTML for the page body into a top level variable.
  $variables['page']              = $variables['page']['#children'];
  $variables['page_bottom'] .= drupal_get_js('footer');

  $variables['head']    = drupal_get_html_head();
  $variables['css']     = drupal_add_css();
  $variables['styles']  = drupal_get_css();
  $variables['scripts'] = drupal_get_js();
}

Proposed resolution

The template_process_html function needs to be removed from Drupal 8.

Remaining tasks

- All the calls to drupal_render need to be replaced with __toString methods on the renderable object.
- Calls to drupal_get_css and drupal_get_js can be moved up to the preprocess layer, since they return renderables.
- Remove $variables['css'] = drupal_add_css(); leftovers?
- Move $variables['page'] = $variables['page']['#children']; into a __toString method?