Enable "system-ui" generic font family

third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 43 matching lines @@
              inLanguage:(id)useNil;
 + (NSFont*)findFontLike:(NSFont*)font
            forCharacter:(UniChar)uc
              inLanguage:(id)useNil;
 @end
 
 namespace blink {
 
 const char* kColorEmojiFontMac = "Apple Color Emoji";
 
+// static
+const AtomicString& FontCache::legacySystemFontFamily() {
+  DEFINE_STATIC_LOCAL(AtomicString, legacySystemFontFamily,
+                      ("BlinkMacSystemFont"));
+  return legacySystemFontFamily;
+}
+
 static void invalidateFontCache() {
   if (!isMainThread()) {
     Platform::current()->mainThread()->getWebTaskRunner()->postTask(
         BLINK_FROM_HERE, WTF::bind(&invalidateFontCache));
     return;
   }
   FontCache::fontCache()->invalidate();
 }
 
 static void fontCacheRegisteredFontsChangedNotificationCallback(
@@ skipping 203 matching lines @@
   std::unique_ptr<FontPlatformData> platformData = wrapUnique(
       new FontPlatformData(platformFont, size, syntheticBold, syntheticItalic,
                            fontDescription.orientation()));
   if (!platformData->typeface()) {
     return nullptr;
   }
   return platformData;
 }
 
 }  // namespace blink