Enable "system-ui" generic font family

third_party/WebKit/Source/platform/fonts/linux/FontCacheLinux.cpp

Index: third_party/WebKit/Source/platform/fonts/linux/FontCacheLinux.cpp
diff --git a/third_party/WebKit/Source/platform/fonts/linux/FontCacheLinux.cpp b/third_party/WebKit/Source/platform/fonts/linux/FontCacheLinux.cpp
index bfc61b7aae54553b1d14c9cfb2f4ac6ce4a4dec0..f38677a9c622ff50372cd46713e22dafc7ba9d65 100644
--- a/third_party/WebKit/Source/platform/fonts/linux/FontCacheLinux.cpp
+++ b/third_party/WebKit/Source/platform/fonts/linux/FontCacheLinux.cpp
@@ -43,6 +43,25 @@ FontCache::FontCache() : m_purgePreventCount(0) {
   }
 }
 
+// The system UI font is localizable. When not localized,
+// FontCache::createTypeface() maps "system-ui" to the Skia default font.
+static AtomicString& mutableSystemFontFamily() {
+  DEFINE_STATIC_LOCAL(AtomicString, systemFontFamily,
+                      (FontFamilyNames::system_ui));
+  return systemFontFamily;
+}
+
+// static
+const AtomicString& FontCache::systemFontFamily(float) {
+  return mutableSystemFontFamily();
+}
+
+// static
+void FontCache::setSystemFontFamily(const char* familyName) {
+  mutableSystemFontFamily() =
+      !familyName || !*familyName ? familyName : FontFamilyNames::system_ui;
+}
+
 void FontCache::getFontForCharacter(
     UChar32 c,
     const char* preferredLocale,