Add font substitution metric for Linux

pdf/preview_mode_client.cc

Index: pdf/preview_mode_client.cc
diff --git a/pdf/preview_mode_client.cc b/pdf/preview_mode_client.cc
index a371b08b591505ee3b1c2699bc4bf548de9d18b7..b3744c93eb7ac81b0de4f9b0d604cd07cfe8c465 100644
--- a/pdf/preview_mode_client.cc
+++ b/pdf/preview_mode_client.cc
@@ -146,6 +146,10 @@ void PreviewModeClient::DocumentHasUnsupportedFeature(
   NOTREACHED();
 }
 
+void PreviewModeClient::FontSubstituted() {
+  NOTREACHED();
+}
+
 void PreviewModeClient::DocumentLoadProgress(uint32_t available,
                                              uint32_t doc_size) {}