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}
 bool WebContentsImpl::IsLoading() const {
   return frame_tree_.IsLoading() &&
         !(ShowingInterstitialPage() && interstitial_page_->pause_throbber());
 }

 bool WebContentsImpl::IsLoading() const {
   return frame_tree_.IsLoading() &&
         !(ShowingInterstitialPage() &&
           GetRenderManager()->interstitial_page()->pause_throbber());
 }
