html: Support selecting <media> element source from children (#39717)

Follow the HTML implemetation and support selection of the 'media'
element loading source from 'source' children elements with validation
processing over their 'media/src/type' attributes.

See https://html.spec.whatwg.org/multipage/#concept-media-load-algorithm

To handle loading with multiple 'source' children was introduced source
children pointer to track the remaining unprocessed 'source' elements
after the resource selection algorithm was invoked.

Testing: Changes in the following tests
with improvements:
-
html/semantics/embedded-content/media-elements/loading-the-media-resource/*
- webgl/tests/conformance/textures/misc/texture-video-transparent.html

with regressions (after enabled 'source' type attribute validation):
- content-security-policy/media-src/media-src*
- mixed-content/gen/top\*audio|video-tag*
- resource-timing/initiator-type/*

Fixes: https://github.com/servo/servo/issues/21481
Fixes: https://github.com/servo/servo/issues/34127

Signed-off-by: Andrei Volykhin <andrei.volykhin@gmail.com>

components/script/dom/html/htmlsourceelement.rs

@@ -105,8 +105,11 @@ impl VirtualMethods for HTMLSourceElement {
 
         // Step 2. If parent is a media element that has no src attribute and whose networkState has
         // the value NETWORK_EMPTY, then invoke that media element's resource selection algorithm.
-        if let Some(media) = parent.downcast::<HTMLMediaElement>() {
-            media.handle_source_child_insertion(can_gc);
+        if parent.is::<HTMLMediaElement>() && std::ptr::eq(&*parent, context.parent) {
+            parent
+                .downcast::<HTMLMediaElement>()
+                .unwrap()
+                .handle_source_child_insertion(self, can_gc);
         }
 
         // Step 3. If parent is a picture element, then for each child of parent's children, if