[0.9] Can't override the css property if the property is already set on the host via custom property

[frankiefu]

x-toolbar.html

<style is="custom-style">
  :root {
    --toolbar-color: blue;
  }
</style>

<dom-module id="x-toolbar">
  <style>
    :host {
      display: block;
      height: 64px;
      background: var(--toolbar-color);
    }
  </style>

  <template>
    <content></content>
  </template>

  <script>
    Polymer({
      is: 'x-toolbar'
    });
  </script>

</dom-module>

app.html

<style>
  x-toolbar {
    background: red;
  }
</style>

<x-toolbar></x-toolbar>

This should make x-toolbar have "red" background but instead it displays as "blue". Also noticed if shadow dom is on (dom=shadow) then the correct color ("blue") is shown.

[sorvell]

Maybe related #1277

[sorvell]

Fixed via 041b732