libservo: Combine LoadStart, HeadParsed, and LoadComplete` messages (#35260)

These will be a single method in the upcoming `WebView` delegate, so it
makes sense to also combine the internal message to match this. In
addition, since `LoadStatus` is now exposed to the API if there is ever
the need to add more statuses or to move to an event-based version, the
API is already set up for this.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

ports/servoshell/desktop/tracing.rs

@@ -148,11 +148,9 @@ mod from_servo {
                 Self::SetClipboardContents(..) => target!("SetClipboardContents"),
                 Self::SetCursor(..) => target!("SetCursor"),
                 Self::NewFavicon(..) => target!("NewFavicon"),
-                Self::HeadParsed(..) => target!("HeadParsed"),
                 Self::HistoryChanged(..) => target!("HistoryChanged"),
                 Self::SetFullscreenState(..) => target!("SetFullscreenState"),
-                Self::LoadStart(..) => target!("LoadStart"),
-                Self::LoadComplete(..) => target!("LoadComplete"),
+                Self::NotifyLoadStatusChanged(..) => target!("NotifyLoadStatusChanged"),
                 Self::Panic(..) => target!("Panic"),
                 Self::GetSelectedBluetoothDevice(..) => target!("GetSelectedBluetoothDevice"),
                 Self::SelectFiles(..) => target!("SelectFiles"),