RenderTextHarfBuzz: Implement font fallback for Win and Linux

ui/gfx/font_fallback_win.h

Index: ui/gfx/font_fallback_win.h
diff --git a/ui/gfx/font_fallback_win.h b/ui/gfx/font_fallback_win.h
index ee675714e7f735eb86999d3ba2f3b1b2d6c0ad19..6c3541267f10a38ebc6097f126707390c5ea96ea 100644
--- a/ui/gfx/font_fallback_win.h
+++ b/ui/gfx/font_fallback_win.h
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "ui/gfx/font.h"
+#include "ui/gfx/font_fallback.h"
 
 namespace gfx {
 
@@ -29,8 +30,6 @@ void GFX_EXPORT ParseFontLinkEntry(const std::string& entry,
 void GFX_EXPORT ParseFontFamilyString(const std::string& family,
                                      std::vector<std::string>* font_names);
 
-}  // namespace internal
-
 // Iterator over linked fallback fonts for a given font. The linked font chain
 // comes from the Windows registry, but gets cached between uses.
 class GFX_EXPORT LinkedFontsIterator {
@@ -77,6 +76,16 @@ class GFX_EXPORT LinkedFontsIterator {
   DISALLOW_COPY_AND_ASSIGN(LinkedFontsIterator);
 };
 
+}  // namespace internal
+
+// Finds a fallback font to render the specified |text| with respect to an
+// initial |font|. Returns the resulting font via out param |result|. Returns
+// |true| if a fallback font was found.
+bool GetUniscribeFallbackFont(const Font& font,
+                              const wchar_t* text,
+                              int text_length,
+                              Font* result);
+
 }  // namespace gfx
 
 #endif  // UI_GFX_FONT_FALLBACK_WIN_H_