linux: handle basic metric-compatibile font fallback

skia/ext/SkFontHost_fontconfig_direct.h

 /* libs/graphics/ports/SkFontHost_fontconfig_direct.h
 **
 ** Copyright 2009, Google Inc.
 **
 ** Licensed under the Apache License, Version 2.0 (the "License");
 ** you may not use this file except in compliance with the License.
 ** You may obtain a copy of the License at
 **
 **     http://www.apache.org/licenses/LICENSE-2.0
 **
@@ skipping 17 matching lines @@
   public:
     FontConfigDirect();
 
     // FontConfigInterface implementation. Thread safe.
     virtual bool Match(std::string* result_family, unsigned* result_fileid,
                        bool fileid_valid, unsigned fileid,
                        const std::string& family, bool* is_bold, bool* is_italic);
     virtual int Open(unsigned fileid);
 
   private:
+    // Return true if font_a and font_b are completely interchangeable;
+    // e.g., there exist free clones of certain well-known fonts.
+    bool IsMetricCompatibleReplacement(const char* font_a, const char* font_b);
+
     SkMutex mutex_;
     std::map<unsigned, std::string> fileid_to_filename_;
     std::map<std::string, unsigned> filename_to_fileid_;
     unsigned next_file_id_;
 };
 
 #endif  // FontConfigDirect_DEFINED