HTML tooltips are not styled correctly

[rslawik]

HTML tooltips in <google-chart> using Polymer 2 and ShadowDOM are not positioned correctly and their styles are missing.

chart.options = {
  tooltip: {
    isHtml: true
  }
}

[wesalvaro]

This looks like a duplicate of #138 which would be fixed by a fix for #26.
I have a fix in #204 but it essentially changes everything, so I need a good reviewer (I also need to add some more tests). I believe that #204 is a much more Polymer-y version that has a bunch of needed changes/refactors. If you want to take a look, that'd be great.