Add "AllArticles" mode to Reader Mode heuristics

This is a preliminary change to enable the desired mode in the back end.

BUG=736168

Review-Url: https://codereview.chromium.org/2961533002
Cr-Commit-Position: refs/heads/master@{#484987}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 916041f..53d19f0 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1107,6 +1107,7 @@
 extern const char kReaderModeHeuristicsDescription[];
 extern const char kReaderModeHeuristicsMarkup[];
 extern const char kReaderModeHeuristicsAdaboost[];
+extern const char kReaderModeHeuristicsAllArticles[];
 extern const char kReaderModeHeuristicsAlwaysOff[];
 extern const char kReaderModeHeuristicsAlwaysOn[];