LibWeb+WebContent: Move Serenity EventLoop and Font plugins into LibWeb

These are exactly what's wanted by headless-browser too, so this saves
us some duplication. LibWeb already links LibCore so it should not
cause any issues for Ladybird.

Userland/Libraries/LibWeb/Platform/EventLoopPluginSerenity.cpp

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "EventLoopPluginSerenity.h"
+#include <AK/Function.h>
+#include <AK/NonnullRefPtr.h>
+#include <LibCore/EventLoop.h>
+#include <LibWeb/Platform/TimerSerenity.h>
+
+namespace Web::Platform {
+
+EventLoopPluginSerenity::EventLoopPluginSerenity() = default;
+EventLoopPluginSerenity::~EventLoopPluginSerenity() = default;
+
+void EventLoopPluginSerenity::spin_until(Function<bool()> goal_condition)
+{
+    Core::EventLoop::current().spin_until(move(goal_condition));
+}
+
+void EventLoopPluginSerenity::deferred_invoke(Function<void()> function)
+{
+    VERIFY(function);
+    Core::deferred_invoke(move(function));
+}
+
+NonnullRefPtr<Timer> EventLoopPluginSerenity::create_timer()
+{
+    return TimerSerenity::create();
+}
+
+}