Track where WebContents are created in order to better understand issue.

content/public/browser/web_contents_source.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/public/browser/web_contents_source.h"
+
+// DEFINE_WEB_CONTENTS_USER_DATA_KEY(WebContentsSource);
+// Can't use the regular macro because this must be CONTENT_EXPORTed.
+template <>
+CONTENT_EXPORT int
+    content::WebContentsUserData<WebContentsSource>::kLocatorKey = 0;
+
+// static
+void WebContentsSource::CreateForWebContentsAndLocation(
+    content::WebContents* contents,
+    const tracked_objects::Location& created_location) {
+  WebContentsSource::CreateForWebContents(contents);
+  WebContentsSource* self = WebContentsSource::FromWebContents(contents);
+  self->created_location_ = created_location;
+}
+
+WebContentsSource::WebContentsSource(content::WebContents* contents) {}