LibWeb: Convert CSS Token::m_unit from StringBuilder to FlyString

This value doesn't change once it's assigned to the Token, so it can be
more lightweight than a StringBuilder.

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

@@ -59,12 +59,12 @@ String Token::to_debug_string() const
     case Type::Percentage:
         builder.append("Percentage: ");
         builder.append(m_value.to_string());
-        builder.append(m_unit.to_string());
+        builder.append('%');
         return builder.to_string();
     case Type::Dimension:
         builder.append("Dimension: ");
         builder.append(m_value.to_string());
-        builder.append(m_unit.to_string());
+        builder.append(m_unit);
         return builder.to_string();
     case Type::Whitespace:
         builder.append("Whitespace");
@@ -140,7 +140,7 @@ String Token::to_debug_string() const
         }
 
         builder.append("', unit: '");
-        builder.append(m_unit.to_string());
+        builder.append(m_unit);
     }
 
     builder.append("' }");

Userland/Libraries/LibWeb/CSS/Parser/Token.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <AK/FlyString.h>
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <math.h>
@@ -132,7 +133,7 @@ public:
     StringView dimension_unit() const
     {
         VERIFY(m_type == Type::Dimension);
-        return m_unit.string_view();
+        return m_unit.view();
     }
     double dimension_value() const
     {
@@ -175,7 +176,7 @@ private:
     Type m_type { Type::Invalid };
 
     StringBuilder m_value;
-    StringBuilder m_unit;
+    FlyString m_unit;
     HashType m_hash_type { HashType::Unrestricted };
     NumberType m_number_type { NumberType::Integer };
     double m_number_value { 0 };

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

@@ -685,7 +685,7 @@ Token Tokenizer::consume_a_numeric_token()
 
         auto unit = consume_a_name();
         VERIFY(!unit.is_empty() && !unit.is_whitespace());
-        token.m_unit.append(unit);
+        token.m_unit = move(unit);
 
         return token;
     }