Add font substitution metric for Linux

pdf/pdfium/pdfium_engine.h

Index: pdf/pdfium/pdfium_engine.h
diff --git a/pdf/pdfium/pdfium_engine.h b/pdf/pdfium/pdfium_engine.h
index 07c15b56e865c9dca7c344e278c244c3028adc45..7826a5dcb57d4cc845fefcd232b4c12930bd5ece 100644
--- a/pdf/pdfium/pdfium_engine.h
+++ b/pdf/pdfium/pdfium_engine.h
@@ -120,6 +120,7 @@ class PDFiumEngine : public PDFEngine,
   void OnDocumentComplete() override;
 
   void UnsupportedFeature(int type);
+  void FontSubstituted();
 
   std::string current_find_text() const { return current_find_text_; }
 
@@ -742,6 +743,21 @@ class ScopedUnsupportedFeature {
 
  private:
   PDFiumEngine* const old_engine_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedUnsupportedFeature);
+};
+
+// Create a local variable of this when calling PDFium functions which can call
+// our global callback when a substitute font is mapped.
+class ScopedSubstFont {
+ public:
+  explicit ScopedSubstFont(PDFiumEngine* engine);
+  ~ScopedSubstFont();
+
+ private:
+  PDFiumEngine* const old_engine_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedSubstFont);
 };
 
 class PDFiumEngineExports : public PDFEngineExports {