Force core-toolbar/core-header to be transparent

Because `core-toolbar` now has a default background (aa8b336327958382adcae3dcfc43a9cf27292ebc) it breaks `core-scroll-header-panel` since the background color was previously transparent (https://github.com/Polymer/polymer/issues/576). This change forces the content inside of `#headerContent` (either `core-toolbar` or `.core-header`) to be transparent again.

Fixes https://github.com/Polymer/polymer/issues/576