Add font substitution metric for Linux

pdf/pdf_engine.h

Index: pdf/pdf_engine.h
diff --git a/pdf/pdf_engine.h b/pdf/pdf_engine.h
index 4d17a403b184ebfa856c76a384a5f9de49319e6f..a394d0271f2d94af72b01fd70c48e7e49150425f 100644
--- a/pdf/pdf_engine.h
+++ b/pdf/pdf_engine.h
@@ -165,6 +165,9 @@ class PDFEngine {
     // Notifies the client that the document has failed to load.
     virtual void DocumentLoadFailed() = 0;
 
+    // Notifies the client that the document has requested substitute fonts.
+    virtual void FontSubstituted() = 0;
+
     virtual pp::Instance* GetPluginInstance() = 0;
 
     // Notifies that an unsupported feature in the PDF was encountered.