Make auxiliary webviews exist in the constellation immediately (#35672)

To allow embedders to interact with webviews as soon as they are
created, we need to ensure that they exist in both the compositor and
the constellation before those interactions happen. #35662 does this
for the compositor, while this patch does this for the constellation.

When a webview opens another webview (via <a target>, <form target>,
window.open(), etc), the embedder creates an “auxiliary” webview,
which previously went as follows:

- script create_auxiliary_browsing_context
- libservo AllowOpeningWebView
- embedder request_open_auxiliary_webview (→ constellation FocusWebView)
- script create_auxiliary_browsing_context
- constellation ScriptNewAuxiliary

In that model, the constellation may receive FocusWebView before it
receives ScriptNewAuxiliary. Now they are created as follows:

- script create_auxiliary_browsing_context
- constellation CreateAuxiliaryWebView
- libservo AllowOpeningWebView
- embedder request_open_auxiliary_webview (→ constellation FocusWebView)
- constellation CreateAuxiliaryWebView
- script create_auxiliary_browsing_context

Since these messages are all synchronous and the constellation will
have set up the webview before handling any new messages, the webview
will always exist by the time we handle the embedder’s FocusWebView.

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

components/constellation/tracing.rs

@@ -167,7 +167,7 @@ mod from_script {
                 Self::SetThrottledComplete(..) => target!("SetThrottledComplete"),
                 Self::ScriptLoadedURLInIFrame(..) => target!("ScriptLoadedURLInIFrame"),
                 Self::ScriptNewIFrame(..) => target!("ScriptNewIFrame"),
-                Self::ScriptNewAuxiliary(..) => target!("ScriptNewAuxiliary"),
+                Self::CreateAuxiliaryWebView(..) => target!("ScriptNewAuxiliary"),
                 Self::ActivateDocument => target!("ActivateDocument"),
                 Self::SetDocumentState(..) => target!("SetDocumentState"),
                 Self::SetLayoutEpoch(..) => target!("SetLayoutEpoch"),