LibWeb: Remove separate Token::m_unit field

Dimension tokens don't make use of the m_value string for anything else,
so we can sneak the unit string in there.

- Token goes from 72 to 64 bytes
- StyleComponentValueRule goes from 80 to 72 bytes

Userland/Libraries/LibWeb/CSS/Parser/Tokenizer.cpp

@@ -845,7 +845,8 @@ Token Tokenizer::consume_a_numeric_token()
         // 2. Consume a name. Set the <dimension-token>’s unit to the returned value.
         auto unit = consume_a_name();
         VERIFY(!unit.is_empty());
-        token.m_unit = move(unit);
+        // NOTE: We intentionally store this in the `value`, to save space.
+        token.m_value = move(unit);
 
         // 3. Return the <dimension-token>.
         return token;