RenderTextHarfBuzz: Implement font fallback for Win and Linux

ui/gfx/font_fallback_win.cc

Index: ui/gfx/font_fallback_win.cc
diff --git a/ui/gfx/font_fallback_win.cc b/ui/gfx/font_fallback_win.cc
index 4426718959b3be04a65160f23b808036d94dc64d..901feb01fb633f8483ac57604359b6979e3add9a 100644
--- a/ui/gfx/font_fallback_win.cc
+++ b/ui/gfx/font_fallback_win.cc
@@ -12,6 +12,7 @@
 #include "base/strings/utf_string_conversions.h"
 #include "base/win/registry.h"
 #include "ui/gfx/font.h"
+#include "ui/gfx/font_fallback.h"
 
 namespace gfx {
 
@@ -186,8 +187,6 @@ void ParseFontFamilyString(const std::string& family,
   }
 }
 
-}  // namespace internal
-
 LinkedFontsIterator::LinkedFontsIterator(Font font)
     : original_font_(font),
       next_font_set_(false),
@@ -243,4 +242,15 @@ const std::vector<Font>* LinkedFontsIterator::GetLinkedFonts() const {
   return fonts;
 }
 
+}  // namespace internal
+
+std::vector<std::string> GetFallbackFontFamilies(std::string font_family) {
+  internal::LinkedFontsIterator linked_fonts(Font(font_family, 10));
+  std::vector<std::string> fallback_fonts;
+  Font current;
+  while (linked_fonts.NextFont(&current))
+    fallback_fonts.push_back(current.GetFontName());
+  return fallback_fonts;
+}
+
 }  // namespace gfx