Abort navigations on 304 responses.

A recent change (https://chromium-review.googlesource.com/1161479)
accidentally resulted in treating 304 responses as downloads. This CL
treats them as ERR_ABORTED instead. This doesn't exactly match old
behavior, which passed them on to the renderer, which then aborted them.

The new code results in correctly restoring the original URL in the
omnibox, and has a shiny new test to prevent future regressions.

Bug: 882270
Change-Id: Ic73dcce9e9596d43327b13acde03b4ed9bd0c82e
Reviewed-on: https://chromium-review.googlesource.com/1252684
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595641}
  explicit RenderProcessKilledObserver(WebContents* web_contents)
      : WebContentsObserver(web_contents) {}

  explicit RenderProcessKilledObserver(WebContents* web_contents)
      : WebContentsObserver(web_contents) {}
