Enable "system-ui" generic font family

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

Index: third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm
diff --git a/third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm b/third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm
index 5956c7a737beead71d6df78f75f6246d807c35e4..a9b9a9eccb1958e2dd3d24e0dc0f1b50ab24f766 100644
--- a/third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm
+++ b/third_party/WebKit/Source/platform/fonts/mac/FontCacheMac.mm
@@ -61,6 +61,19 @@ namespace blink {
 
 const char* kColorEmojiFontMac = "Apple Color Emoji";
 
+// static
+const AtomicString& FontCache::legacySystemFontFamily() {
+  DEFINE_STATIC_LOCAL(AtomicString, legacySystemFontFamily,
+                      ("BlinkMacSystemFont"));
+  return legacySystemFontFamily;
+}
+
+// static
+const AtomicString& FontCache::systemFontFamily(float) {
+  // TODO(kojii): Get the actual font name for the size.
+  return FontFamilyNames::system_ui;
+}
+
 static void invalidateFontCache() {
   if (!isMainThread()) {
     Platform::current()->mainThread()->getWebTaskRunner()->postTask(