LibWeb+WebContent: Add abstraction layer for generic font families

Instead of hard-coding the names of system fonts to use for the CSS
generic fonts (like "sans-serif", "monospace", etc.) we now call out
to a Platform::FontPlugin and ask for the generic names.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -352,6 +352,7 @@ set(SOURCES
     Painting/StackingContext.cpp
     Painting/TextPaintable.cpp
     Platform/EventLoopPlugin.cpp
+    Platform/FontPlugin.cpp
     Platform/Timer.cpp
     RequestIdleCallback/IdleDeadline.cpp
     ResizeObserver/ResizeObserver.cpp

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -27,6 +27,7 @@
 #include <LibWeb/FontCache.h>
 #include <LibWeb/HTML/HTMLHtmlElement.h>
 #include <LibWeb/Loader/ResourceLoader.h>
+#include <LibWeb/Platform/FontPlugin.h>
 #include <stdio.h>
 
 namespace Web::CSS {
@@ -1029,28 +1030,39 @@ void StyleComputer::compute_font(StyleProperties& style, DOM::Element const* ele
         return {};
     };
 
-    // FIXME: Replace hard-coded font names with a relevant call to FontDatabase.
-    // Currently, we cannot request the default font's name, or request it at a specific size and weight.
-    // So, hard-coded font names it is.
     auto find_generic_font = [&](ValueID font_id) -> RefPtr<Gfx::Font> {
+        Platform::GenericFont generic_font {};
         switch (font_id) {
         case ValueID::Monospace:
         case ValueID::UiMonospace:
+            generic_font = Platform::GenericFont::Monospace;
             monospace = true;
-            return find_font("Csilla");
+            break;
         case ValueID::Serif:
-            return find_font("Roman");
+            generic_font = Platform::GenericFont::Serif;
+            break;
         case ValueID::Fantasy:
-            return find_font("Comic Book");
+            generic_font = Platform::GenericFont::Fantasy;
+            break;
         case ValueID::SansSerif:
+            generic_font = Platform::GenericFont::SansSerif;
+            break;
         case ValueID::Cursive:
+            generic_font = Platform::GenericFont::Cursive;
+            break;
         case ValueID::UiSerif:
+            generic_font = Platform::GenericFont::UiSerif;
+            break;
         case ValueID::UiSansSerif:
+            generic_font = Platform::GenericFont::UiSansSerif;
+            break;
         case ValueID::UiRounded:
-            return find_font("Katica");
+            generic_font = Platform::GenericFont::UiRounded;
+            break;
         default:
             return {};
         }
+        return find_font(Platform::FontPlugin::the().generic_font_name(generic_font));
     };
 
     RefPtr<Gfx::Font> found_font;

Userland/Libraries/LibWeb/Platform/FontPlugin.cpp

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Platform/FontPlugin.h>
+
+namespace Web::Platform {
+
+static FontPlugin* s_the;
+
+FontPlugin& FontPlugin::the()
+{
+    VERIFY(s_the);
+    return *s_the;
+}
+
+void FontPlugin::install(FontPlugin& plugin)
+{
+    VERIFY(!s_the);
+    s_the = &plugin;
+}
+
+FontPlugin::~FontPlugin() = default;
+
+}

Userland/Libraries/LibWeb/Platform/FontPlugin.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+
+namespace Web::Platform {
+
+enum class GenericFont {
+    Cursive,
+    Fantasy,
+    Monospace,
+    SansSerif,
+    Serif,
+    UiMonospace,
+    UiRounded,
+    UiSansSerif,
+    UiSerif,
+    __Count,
+};
+
+class FontPlugin {
+public:
+    static FontPlugin& the();
+    static void install(FontPlugin&);
+
+    virtual ~FontPlugin();
+
+    virtual String generic_font_name(GenericFont) = 0;
+};
+
+}