Don't show current RenderWidgetHostView while interstitial is showing.

Also moves interstitial page tracking from RenderFrameHostManager to
WebContents, since interstitial pages are not frame-specific. This was
necessary for subframes to detect if an interstitial page is showing.

BUG=729105
TEST=See comment 13 of bug for repro steps
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2938313002
Cr-Commit-Position: refs/heads/master@{#480117}
RenderFrameProxyHost* RenderFrameHostManager::GetProxyToParent() {
  if (frame_tree_node_->IsMainFrame())
    return nullptr;

  return GetRenderFrameProxyHost(frame_tree_node_->parent()
                                     ->render_manager()
                                     ->current_frame_host()
                                     ->GetSiteInstance());
}

RenderFrameProxyHost* RenderFrameHostManager::GetProxyToParent() {
  if (frame_tree_node_->IsMainFrame())
    return nullptr;

  return GetRenderFrameProxyHost(frame_tree_node_->parent()
                                     ->render_manager()
                                     ->current_frame_host()
                                     ->GetSiteInstance());
}
