LibWeb: Implement navigator.deviceMemory

Fixes at least 2 subtests in wpt/device-memory.

Userland/Libraries/LibWeb/HTML/NavigatorDeviceMemory.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2024, Jelle Raaijmakers <jelle@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/BuiltinWrappers.h>
+#include <LibCore/System.h>
+#include <LibWeb/WebIDL/Types.h>
+
+namespace Web::HTML {
+
+class NavigatorDeviceMemoryMixin {
+public:
+    // https://www.w3.org/TR/device-memory/#computing-device-memory-value
+    WebIDL::Double device_memory() const
+    {
+        // The value is calculated by using the actual device memory in MiB then rounding it to the
+        // nearest number where only the most signicant bit can be set and the rest are zeros
+        // (nearest power of two).
+        auto memory_in_bytes = Core::System::physical_memory_bytes();
+        auto memory_in_mib = memory_in_bytes / MiB;
+        auto required_bits = AK::count_required_bits(memory_in_mib);
+        auto lower_memory_in_mib = static_cast<u64>(1) << (required_bits - 1);
+        auto upper_memory_in_mib = static_cast<u64>(1) << required_bits;
+        auto rounded_memory_in_mib = upper_memory_in_mib - memory_in_mib <= memory_in_mib - lower_memory_in_mib
+            ? upper_memory_in_mib
+            : lower_memory_in_mib;
+
+        // Then dividing that number by 1024.0 to get the value in GiB.
+        auto memory_in_gib = static_cast<WebIDL::Double>(rounded_memory_in_mib) / 1024.0;
+
+        // An upper bound and a lower bound should be set on the list of values.
+        return AK::clamp(memory_in_gib, 1.0, 4.0);
+    }
+};
+
+}