MacViews: Implement FormatType::operator< on Mac.

ui/base/clipboard/clipboard_mac.mm

Index: ui/base/clipboard/clipboard_mac.mm
diff --git a/ui/base/clipboard/clipboard_mac.mm b/ui/base/clipboard/clipboard_mac.mm
index 63c1edf7cf5de8579b8912b73e7323c9ff45d9f7..bfc3df6a9bea31700b4293833b605dfb0dd37dc0 100644
--- a/ui/base/clipboard/clipboard_mac.mm
+++ b/ui/base/clipboard/clipboard_mac.mm
@@ -89,6 +89,10 @@ Clipboard::FormatType Clipboard::FormatType::Deserialize(
   return FormatType(base::SysUTF8ToNSString(serialization));
 }
 
+bool Clipboard::FormatType::operator<(const FormatType& other) const {
+  return [data_ compare:other.data_] == NSOrderedAscending;
+}
+
 // Various predefined FormatTypes.
 // static
 Clipboard::FormatType Clipboard::GetFormatType(