Fix issue with pending NavigationEntry being discarded incorrectly

This CL fixes an issue where we would attempt to discard a pending
NavigationEntry when a cross-process navigation to this NavigationEntry
is interrupted by another navigation to the same NavigationEntry.

BUG=760342,797656,796135

Change-Id: I204deff1efd4d572dd2e0b20e492592d48d787d9
Reviewed-on: https://chromium-review.googlesource.com/850877
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528611}
  explicit RenderFrameHostDestructionObserver(RenderFrameHost* rfh)
      : WebContentsObserver(WebContents::FromRenderFrameHost(rfh)),
        message_loop_runner_(new MessageLoopRunner),
        deleted_(false),
        render_frame_host_(rfh) {}

  explicit RenderFrameHostDestructionObserver(RenderFrameHost* rfh)
      : WebContentsObserver(WebContents::FromRenderFrameHost(rfh)),
        message_loop_runner_(new MessageLoopRunner),
        deleted_(false),
        render_frame_host_(rfh) {}
