Enable "system-ui" generic font family

third_party/WebKit/Source/platform/fonts/FontCache.h

Index: third_party/WebKit/Source/platform/fonts/FontCache.h
diff --git a/third_party/WebKit/Source/platform/fonts/FontCache.h b/third_party/WebKit/Source/platform/fonts/FontCache.h
index b97a5a8da02e772732f5ea2dcfeaedefa363aedd..9e6dbc8b5ea622956939b6d640f3adeb44e28ee4 100644
--- a/third_party/WebKit/Source/platform/fonts/FontCache.h
+++ b/third_party/WebKit/Source/platform/fonts/FontCache.h
@@ -115,6 +115,15 @@ class PLATFORM_EXPORT FontCache {
   SkFontMgr* fontManager() { return m_fontManager.get(); }
   static void setFontManager(const sk_sp<SkFontMgr>&);
 
+#if !OS(MACOSX)
+  static const AtomicString& systemFontFamily();
+#else
+  static const AtomicString& legacySystemFontFamily();
+#endif
+#if OS(LINUX)
+  static void setSystemFontFamily(const char*);
+#endif
+
 #if OS(WIN)
   static bool antialiasedTextEnabled() { return s_antialiasedTextEnabled; }
   static bool lcdTextEnabled() { return s_lcdTextEnabled; }