Polish CPDF_FontSubsetter and its embedder tests

Polish CPDF_FontSubsetter and its embedder tests one more time by
performing the following:
1. Add comments mentioning that Type 1 font subsetting is not supported,
   and the reason why.
2. Fix PrintTo() not being used.
   https://pdfium-review.googlesource.com/c/pdfium/+/143773 modified the
   obj param type, but this caused gtest to not be able to look up the
   function. The obj type must be a const-ref.

Fixed: 476127152
Change-Id: Ia620844b20d55dbccd749b4b2e9049b33ea8bf94
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/143990
Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

core/fpdfapi/edit/cpdf_fontsubsetter.cpp

diff --git a/core/fpdfapi/edit/cpdf_fontsubsetter.cpp b/core/fpdfapi/edit/cpdf_fontsubsetter.cpp
index 381ce29..490d401 100644
--- a/core/fpdfapi/edit/cpdf_fontsubsetter.cpp
+++ b/core/fpdfapi/edit/cpdf_fontsubsetter.cpp
@@ -280,6 +280,9 @@
       continue;
     }
 
+    // Internally, all embedded font file streams are set as "FontFile" or
+    // "FontFile2". HarfBuzz does not support subsetting Type 1 fonts, so fonts
+    // with "FontFile" are ignored.
     RetainPtr<const CPDF_Stream> font_stream =
         descriptor->GetStreamFor("FontFile2");
     if (!font_stream) {

core/fpdfapi/edit/cpdf_fontsubsetter.h

diff --git a/core/fpdfapi/edit/cpdf_fontsubsetter.h b/core/fpdfapi/edit/cpdf_fontsubsetter.h
index 954301b..be5633a 100644
--- a/core/fpdfapi/edit/cpdf_fontsubsetter.h
+++ b/core/fpdfapi/edit/cpdf_fontsubsetter.h
@@ -26,6 +26,7 @@
 // in order to subset new embedded fonts. CPDF_FontSubsetter only creates new
 // PDF objects and does not modify any existing PDF objects. Use during saving
 // to create objects that should override existing objects when writing the PDF.
+// CPDF_FontSubsetter does not support Type 1 fonts.
 class CPDF_FontSubsetter {
  public:
   explicit CPDF_FontSubsetter(CPDF_Document* doc);

core/fpdfapi/edit/cpdf_fontsubsetter_embeddertest.cpp

diff --git a/core/fpdfapi/edit/cpdf_fontsubsetter_embeddertest.cpp b/core/fpdfapi/edit/cpdf_fontsubsetter_embeddertest.cpp
index 93d6edd..500ca02 100644
--- a/core/fpdfapi/edit/cpdf_fontsubsetter_embeddertest.cpp
+++ b/core/fpdfapi/edit/cpdf_fontsubsetter_embeddertest.cpp
@@ -276,7 +276,7 @@
 }  // namespace
 
 // Prints overrides nicely for debugging purposes.
-void PrintTo(const CPDF_Object* obj, std::ostream* os) {
+void PrintTo(const RetainPtr<const CPDF_Object>& obj, std::ostream* os) {
   if (!obj) {
     *os << "nullptr";
     return;