style: Include the 'unsafe' keyword in serializations of css-align properties.

Previously we'd omit it since it was merely an explicit way of requesting the
default behavior.  But the spec has changed such that it's not necessarily
equivalent to the default anymore:
https://drafts.csswg.org/css-align/#overflow-values

(Technically the behaviors are probably still equivalent in our implementation,
pending bug 1451380, but we don't have to publicize that via our
serialization.)

Differential Revision: https://phabricator.services.mozilla.com/D14599

components/style/values/specified/align.rs

@@ -92,8 +92,7 @@ impl ToCss for AlignFlags {
                 dest.write_char(' ')?;
             },
             AlignFlags::SAFE => dest.write_str("safe ")?,
-            // Don't serialize "unsafe", since it's the default.
-            AlignFlags::UNSAFE => {},
+            AlignFlags::UNSAFE => dest.write_str("unsafe ")?,
             _ => {
                 debug_assert_eq!(extra_flags, AlignFlags::empty());
             },