style(subheader): width styling with scrolling

[ThomasBurleson]

Demo with Subheader shows subheader not full-width when elevated due to scrolling.

[rschmukler]

This is a byproduct of achieving a mobile-performant sticky subheader.

[robertmesserle]

I believe a wrapper div that does not scroll should handle this.

<md-content>
  <div class="content-inner">
    <!-- sticky headers live here -->
  </div>
</md-content>