Add log to DCHECK to help in debugging its triggering

TBR=johnchen@chromium.org

Bug: 852616
Change-Id: I8046cc162d1cb39d9809bd2bcd3a09b3a29212b8
Reviewed-on: https://chromium-review.googlesource.com/1100125Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567120}

chrome/browser/offline_pages/recent_tab_helper.cc

@@ -467,7 +467,9 @@ void RecentTabHelper::SavePageCallback(SnapshotProgressInfo* snapshot_info,
                                        int64_t offline_id) {
   DCHECK((snapshot_info->IsForLastN() &&
           snapshot_info->request_id == OfflinePageModel::kInvalidOfflineId) ||
-         snapshot_info->request_id == offline_id);
+         snapshot_info->request_id == offline_id)
+      << "IsForLastN: " << snapshot_info->IsForLastN()
+      << ", request_id: " << snapshot_info->request_id;
   // Store the assigned offline_id (for downloads case it will already contain
   // the same value).
   snapshot_info->request_id = offline_id;