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..729b3e0afd4d77dfb16b7a7a853a85b3ab1b3d58 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_;

[Lei Zhang, 2016/10/11 22:35:59]

I'm worried we set this but never unset it. There is only 1 PDF plugin process,
and it contains multiple engines. If engine A reaches here first, and then
engine B reaches here, and then engine B gets destroyed... we can end up with a
dangling pointer if engine A then reaches MapFont() somehow.

[npm, 2016/10/12 18:19:52]

I see that. But this is how g_last_instance_id works. Maybe the best way is to
set - unset the client on the same method.

[Lei Zhang, 2016/10/12 18:46:58]

However, |g_last_instance_id| is an int, and having an invalid value does not
cause potential use-after-frees.

   bool needs_password = false;
   if (TryLoadingDoc(std::string(), &needs_password)) {
     ContinueLoadingDocument(std::string());