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}
FrameTreeNode* RenderFrameHostManager::GetOuterDelegateNode() {
  int outer_contents_frame_tree_node_id =
      delegate_->GetOuterDelegateFrameTreeNodeId();
  return FrameTreeNode::GloballyFindByID(outer_contents_frame_tree_node_id);
}

FrameTreeNode* RenderFrameHostManager::GetOuterDelegateNode() {
  int outer_contents_frame_tree_node_id =
      delegate_->GetOuterDelegateFrameTreeNodeId();
  return FrameTreeNode::GloballyFindByID(outer_contents_frame_tree_node_id);
}
