Create new image cache per document (#36832)

Rather than sharing the full image cache in a script_thread, the image
cache is now unique per document. This ensures that CSP factors no
longer affect whether the image is retrieved from the cache incorrectly.

To do so, the thread_pool is shared across all caches, but the store is
fresh. Except for the place_holder{image,url}, which are cloned. That's
because the `rippy_data` is only available in the constellation and no
longer accessible at the point that we need to create the document in
the script_thread.

Contrary to the description in #36505, the script_thread still has an
image_cache for this reason: so it has access to the store and
thread_pool to clone it.

With these changes, the two CSP tests no longer flake. Confirmed with
running the following commmand:

```
./mach test-wpt tests/wpt/tests/content-security-policy/generic/ --rerun=10
```

Fixes #36505

Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>

components/shared/net/image_cache.rs

@@ -141,4 +141,10 @@ pub trait ImageCache: Sync + Send {
 
     /// Inform the image cache about a response for a pending request.
     fn notify_pending_response(&self, id: PendingImageId, action: FetchResponseMsg);
+
+    /// Create new image cache based on this one, while reusing the existing thread_pool.
+    fn create_new_image_cache(
+        &self,
+        compositor_api: CrossProcessCompositorApi,
+    ) -> Arc<dyn ImageCache>;
 }