Add font substitution metric for Linux

pdf/pdfium/pdfium_engine.cc

Index: pdf/pdfium/pdfium_engine.cc
diff --git a/pdf/pdfium/pdfium_engine.cc b/pdf/pdfium/pdfium_engine.cc
index f3ae32c109488f52f10438a9ddf030c339d2c431..5cb8cfd67551d424d9e772cfea634cbc75764470 100644
--- a/pdf/pdfium/pdfium_engine.cc
+++ b/pdf/pdfium/pdfium_engine.cc
@@ -110,6 +110,8 @@ const int32_t kMaxProgressivePaintTimeMs = 300;
 // process.
 const int32_t kMaxInitialProgressivePaintTimeMs = 250;
 
+PDFEngine::Client* g_client_for_fontmapper = nullptr;
+
 std::vector<uint32_t> GetPageNumbersFromPrintPageNumberRange(
     const PP_PrintPageNumberRange_Dev* page_ranges,
     uint32_t page_range_count) {
@@ -259,6 +261,9 @@ void* MapFont(struct _FPDF_SYSFONTINFO*, int weight, int italic,
     return nullptr;
   }
 
+  if (g_client_for_fontmapper)
+    g_client_for_fontmapper->FontSubstituted();
+
   PP_Resource font_resource = pp::PDF::GetFontFileWithFallback(
       pp::InstanceHandle(g_last_instance_id),
       &description.pp_font_description(),
@@ -2447,6 +2452,7 @@ void PDFiumEngine::LoadDocument() {
     return;
 
   ScopedUnsupportedFeature scoped_unsupported_feature(this);
+  g_client_for_fontmapper = client_;
   bool needs_password = false;
   if (TryLoadingDoc(std::string(), &needs_password)) {
     ContinueLoadingDocument(std::string());