LibJS+LibWeb: Spin event loop via VM::CustomData abstraction

Instead of calling Core::EventLoop directly, LibJS now has a virtual
function on VM::CustomData for customizing this behavior.

We use this in LibWeb to plumb the spin request through to the
PlatformEventPlugin.

Userland/Libraries/LibJS/Runtime/Completion.cpp

@@ -97,8 +97,11 @@ ThrowCompletionOr<Value> await(VM& vm, Value value)
     //        by letting the event loop spin until our promise is no longer pending, and then synchronously
     //        running all queued promise jobs.
     // Note: This is not used by LibJS itself, and is performed for the embedder (i.e. LibWeb).
-    if (Core::EventLoop::has_been_instantiated())
-        Core::EventLoop::current().spin_until([&] { return success.has_value(); });
+    if (auto* custom_data = vm.custom_data()) {
+        custom_data->spin_event_loop_until([&] {
+            return success.has_value();
+        });
+    }
 
     // 8. Remove asyncContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
     // NOTE: Since we don't push any EC, this step is not performed.

Userland/Libraries/LibJS/Runtime/VM.h

@@ -34,6 +34,8 @@ class VM : public RefCounted<VM> {
 public:
     struct CustomData {
         virtual ~CustomData() = default;
+
+        virtual void spin_event_loop_until(Function<bool()> goal_condition) = 0;
     };
 
     static NonnullRefPtr<VM> create(OwnPtr<CustomData> = {});

Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp

@@ -21,6 +21,7 @@
 #include <LibWeb/HTML/Scripting/Environments.h>
 #include <LibWeb/HTML/Scripting/ExceptionReporter.h>
 #include <LibWeb/HTML/Window.h>
+#include <LibWeb/Platform/EventLoopPlugin.h>
 
 namespace Web::Bindings {
 
@@ -413,4 +414,9 @@ NonnullOwnPtr<JS::ExecutionContext> create_a_new_javascript_realm(JS::VM& vm, Fu
     return realm_execution_context;
 }
 
+void WebEngineCustomData::spin_event_loop_until(Function<bool()> goal_condition)
+{
+    Platform::EventLoopPlugin::the().spin_until(move(goal_condition));
+}
+
 }

Userland/Libraries/LibWeb/Bindings/MainThreadVM.h

@@ -20,6 +20,8 @@ namespace Web::Bindings {
 struct WebEngineCustomData final : public JS::VM::CustomData {
     virtual ~WebEngineCustomData() override = default;
 
+    virtual void spin_event_loop_until(Function<bool()> goal_condition) override;
+
     HTML::EventLoop event_loop;
 
     // FIXME: These should only be on similar-origin window agents, but we don't currently differentiate agent types.