[TEST] Check when the BackForward PageTransition is dropped.

content/browser/frame_host/navigation_handle_impl.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/frame_host/navigation_handle_impl.h"
 
 #include <iterator>
 
 #include "base/logging.h"
 #include "base/metrics/histogram_macros.h"
@@ skipping 726 matching lines @@
     const FrameHostMsg_DidCommitProvisionalLoad_Params& params,
     bool navigation_entry_committed,
     bool did_replace_entry,
     const GURL& previous_url,
     NavigationType navigation_type,
     RenderFrameHostImpl* render_frame_host) {
   DCHECK(!render_frame_host_ || render_frame_host_ == render_frame_host);
   DCHECK_EQ(frame_tree_node_, render_frame_host->frame_tree_node());
   CHECK_EQ(url_, params.url);
 
+  // Will crash when the transition was FORWARD_BACK and becomes something else
+  // for mysterious reasons.
+  CHECK(!(transition_ & ui::PAGE_TRANSITION_FORWARD_BACK) ||
+        params.transition & ui::PAGE_TRANSITION_FORWARD_BACK);

[Charlie Reis, 2017/06/27 03:59:29]

Are you worried at all about the other direction?  It seems odd to allow cases
that FORWARD_BACK wasn't initially true but becomes true during the navigation. 
(That is, I'm curious why this is an A => B check instead of A == B.)

+
   did_replace_entry_ = did_replace_entry;
   method_ = params.method;
   has_user_gesture_ = (params.gesture == NavigationGestureUser);
   transition_ = params.transition;
   should_update_history_ = params.should_update_history;
   render_frame_host_ = render_frame_host;
   previous_url_ = previous_url;
   base_url_ = params.base_url;
   socket_address_ = params.socket_address;
   navigation_type_ = navigation_type;
@@ skipping 403 matching lines @@
   if (new_site_url == site_url_)
     return;
 
   // When redirecting cross-site, stop telling the speculative
   // RenderProcessHost to expect a navigation commit.
   SetExpectedProcess(nullptr);
   site_url_ = new_site_url;
 }
 
 }  // namespace content