script: Chain up keyboard scrolling to parent <iframe>s (#39469)

When an `<iframe>` cannot scroll because the size of the frame is
greater than or
equal to the size of page contents, chain up the keyboard scroll
operation to the parent frame.

Testing: A new Servo-only WPT tests is added, though needs to be
manually
run with `--product servodriver`.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Delan Azabani <dazabani@igalia.com>

components/script/messaging.rs

@@ -101,6 +101,7 @@ impl MixedMessage {
                 ScriptThreadMessage::SendImageKeysBatch(..) => None,
                 ScriptThreadMessage::PreferencesUpdated(..) => None,
                 ScriptThreadMessage::NoLongerWaitingOnAsychronousImageUpdates(_) => None,
+                ScriptThreadMessage::ForwardKeyboardScroll(id, _) => Some(*id),
             },
             MixedMessage::FromScript(inner_msg) => match inner_msg {
                 MainThreadScriptMsg::Common(CommonScriptMsg::Task(_, _, pipeline_id, _)) => {