Include WebViewId into EmbedderMsg variants where possible (#35211)

`EmbedderMsg` was previously paired with an implicit
`Option<WebViewId>`, even though almost all variants were either always
`Some` or always `None`, depending on whether there was a `WebView
involved.

This patch adds the `WebViewId` to as many `EmbedderMsg` variants as
possible, so we can call their associated `WebView` delegate methods
without needing to check and unwrap the `Option`. In many cases, this
required more changes to plumb through the `WebViewId`.

Notably, all `Request`s now explicitly need a `WebView` or not, in order
to ensure that it is passed when appropriate.

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

components/net/tests/filemanager_thread.rs

@@ -7,6 +7,7 @@ use std::io::Read;
 use std::path::PathBuf;
 use std::sync::Arc;
 
+use base::id::TEST_WEBVIEW_ID;
 use embedder_traits::FilterPattern;
 use ipc_channel::ipc;
 use net::filemanager_thread::FileManager;
@@ -44,6 +45,7 @@ fn test_filemanager() {
         // Try to select a dummy file "components/net/tests/test.jpeg"
         let (tx, rx) = ipc::channel().unwrap();
         filemanager.handle(FileManagerThreadMsg::SelectFile(
+            TEST_WEBVIEW_ID,
             patterns.clone(),
             tx,
             origin.clone(),