Add font substitution metric for Linux

pdf/out_of_process_instance.h

Index: pdf/out_of_process_instance.h
diff --git a/pdf/out_of_process_instance.h b/pdf/out_of_process_instance.h
index a998537dfd1e27548cf580d50715c68c928afb51..c4ab2daf36237b1ff287ec473f5addff854606f4 100644
--- a/pdf/out_of_process_instance.h
+++ b/pdf/out_of_process_instance.h
@@ -128,6 +128,7 @@ class OutOfProcessInstance : public pp::Instance,
   void DocumentPaintOccurred() override;
   void DocumentLoadComplete(int page_count) override;
   void DocumentLoadFailed() override;
+  void FontSubstituted() override;
   pp::Instance* GetPluginInstance() override;
   void DocumentHasUnsupportedFeature(const std::string& feature) override;
   void DocumentLoadProgress(uint32_t available, uint32_t doc_size) override;
@@ -309,6 +310,10 @@ class OutOfProcessInstance : public pp::Instance,
   // the stats if a feature shows up many times per document.
   std::set<std::string> unsupported_features_reported_;
 
+  // Keeps track of whether font substitution has been reported, so we avoid
+  // spamming the stats if a document requested multiple substitutes.
+  bool font_substitution_reported_;
+
   // Number of pages in print preview mode, 0 if not in print preview mode.
   int print_preview_page_count_;
   std::vector<int> print_preview_page_numbers_;