PlzNavigate: identify same-page browser-initiated navigation.

third_party/WebKit/Source/core/loader/FrameLoader.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights
  * reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  * Copyright (C) 2008 Alp Toker <alp@atoker.com>
  * Copyright (C) Research In Motion Limited 2009. All rights reserved.
  * Copyright (C) 2011 Kris Jordan <krisjordan@gmail.com>
  * Copyright (C) 2011 Google Inc. All rights reserved.
@@ skipping 1165 matching lines @@
           newLoadType, false, request.triggeringEvent()));
       if (shouldTreatURLAsSameAsCurrent(url))
         newLoadType = FrameLoadTypeReplaceCurrentItem;
     }
 
     loadInSameDocument(url, stateObject, newLoadType, historyLoadType,
                        request.clientRedirect(), request.originDocument());
     return;
   }
 
+  // The navigation has not been detected to be same document. If the request
+  // was declared to be, it means that a renderer-side navigation occurs in
+  // between. In that case, this navigation is dropped.

[clamy, 2016/12/20 15:16:22]

Can you prefix the comment with // PlzNavigate ?
I would also rephrase it as the following:
// If the loader classifies this navigation as a different document navigation
while the browser intended the navigation to be same-document, drop the
navigation. A different navigation must have committed while the IPC was sent.

[arthursonzogni, 2016/12/21 10:11:08]

Done.

+  if (request.resourceRequest().isSameDocumentNavigation())
+    return;
+
   startLoad(request, newLoadType, policy);
 }
 
 SubstituteData FrameLoader::defaultSubstituteDataForURL(const KURL& url) {
   if (!shouldTreatURLAsSrcdocDocument(url))
     return SubstituteData();
   String srcdoc = m_frame->deprecatedLocalOwner()->fastGetAttribute(srcdocAttr);
   DCHECK(!srcdoc.isNull());
   CString encodedSrcdoc = srcdoc.utf8();
   return SubstituteData(
@@ skipping 763 matching lines @@
                          m_documentLoader ? m_documentLoader->url() : String());
   return tracedValue;
 }
 
 inline void FrameLoader::takeObjectSnapshot() const {
   TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID("loading", "FrameLoader", this,
                                       toTracedValue());
 }
 
 }  // namespace blink