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..7bdc2a3a9eab4514e30d059c5600fe7b58d855e0 100644
--- a/third_party/WebKit/Source/platform/fonts/linux/FontCacheLinux.cpp
+++ b/third_party/WebKit/Source/platform/fonts/linux/FontCacheLinux.cpp
@@ -43,6 +43,22 @@ FontCache::FontCache() : m_purgePreventCount(0) {
   }
 }
 
+static AtomicString& mutableSystemFontFamily() {
+  DEFINE_STATIC_LOCAL(AtomicString, systemFontFamily, ());
+  return systemFontFamily;
+}
+
+// static
+const AtomicString& FontCache::systemFontFamily() {
+  return mutableSystemFontFamily();
+}
+
+// static
+void FontCache::setSystemFontFamily(const char* familyName) {
+  DCHECK(familyName && *familyName);
+  mutableSystemFontFamily() = familyName;
+}
+
 void FontCache::getFontForCharacter(
     UChar32 c,
     const char* preferredLocale,