LibWeb: Add CSS ValueListStyleValue

As the new CSS parser tokenizes its input, we can no longer easily
rely on a StringStyleValue for multi-value properties. (eg, border)
ValueListStyleValue lets us wrap all of the ComponentValues that
the Parser produced for one declaration, as a single StyleValue, to
then be parsed into StyleValues by the StyleResolver.

Originally, I wanted it to be a list of StyleValues, but several
properties use syntax that makes use of non-StyleValue tokens, eg:
```css
/* Syntax using a / */
font: 12px/14px sans-serif;
/* Multiple values separated by commas */
background: url(catdog.png), url(another-image.jpg), blue;
```
Passing the ComponentValue tokens themselves means that all that
information is carried over. The alternative might be to create a
StyleValue subclass for each property and parse them fully inside
the Parser. (eg, `FontStyleValue`)

I decided against `ListStyleValue` as a name, to avoid confusion
with list styles. It's not ideal, but names are hard.

Userland/Libraries/LibWeb/CSS/StyleValue.cpp

@@ -1,11 +1,14 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, Sam Atkins <atkinssj@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <AK/ByteBuffer.h>
+#include <AK/Vector.h>
 #include <LibGfx/Palette.h>
+#include <LibWeb/CSS/Parser/StyleComponentValueRule.h>
 #include <LibWeb/CSS/StyleValue.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/InProcessWebView.h>
@@ -169,4 +172,23 @@ void ImageStyleValue::resource_did_load()
         m_document->browsing_context()->set_needs_display({});
 }
 
+ValueListStyleValue::ValueListStyleValue(Vector<StyleComponentValueRule>&& values)
+    : StyleValue(Type::ValueList)
+    , m_values(move(values))
+{
+}
+
+String ValueListStyleValue::to_string() const
+{
+    StringBuilder builder;
+    builder.appendff("List[{}](", m_values.size());
+    for (auto& value : m_values) {
+        builder.append(value.to_debug_string());
+        builder.append(",");
+    }
+
+    builder.append(")");
+    return builder.to_string();
+}
+
 }

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -1,12 +1,14 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
  * Copyright (c) 2021, Tobias Christiansen <tobi@tobyase.de>
+ * Copyright (c) 2021, Sam Atkins <atkinssj@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <AK/NonnullRefPtrVector.h>
 #include <AK/RefCounted.h>
 #include <AK/RefPtr.h>
 #include <AK/String.h>
@@ -16,6 +18,7 @@
 #include <LibGfx/Bitmap.h>
 #include <LibGfx/Color.h>
 #include <LibWeb/CSS/Length.h>
+#include <LibWeb/CSS/Parser/StyleComponentValueRule.h>
 #include <LibWeb/CSS/PropertyID.h>
 #include <LibWeb/CSS/ValueID.h>
 #include <LibWeb/Forward.h>
@@ -219,6 +222,7 @@ public:
         Position,
         CustomProperty,
         Numeric,
+        ValueList,
     };
 
     Type type() const { return m_type; }
@@ -233,6 +237,7 @@ public:
     bool is_position() const { return type() == Type::Position; }
     bool is_custom_property() const { return type() == Type::CustomProperty; }
     bool is_numeric() const { return type() == Type::Numeric; }
+    bool is_value_list() const { return type() == Type::ValueList; }
 
     virtual String to_string() const = 0;
     virtual Length to_length() const { return Length::make_auto(); }
@@ -460,6 +465,21 @@ private:
     RefPtr<Gfx::Bitmap> m_bitmap;
 };
 
+class ValueListStyleValue final : public StyleValue {
+public:
+    static NonnullRefPtr<ValueListStyleValue> create(Vector<StyleComponentValueRule>&& values) { return adopt_ref(*new ValueListStyleValue(move(values))); }
+    virtual ~ValueListStyleValue() override { }
+
+    virtual String to_string() const override;
+
+    Vector<StyleComponentValueRule> const& values() const { return m_values; }
+
+private:
+    ValueListStyleValue(Vector<StyleComponentValueRule>&&);
+
+    Vector<StyleComponentValueRule> m_values;
+};
+
 inline CSS::ValueID StyleValue::to_identifier() const
 {
     if (is_identifier())