Add WebSocket connection shutdown closure support

Summary:
This CL enhances the `EmbeddedTestServer` and `WebSocketConnection`
classes by adding support for registering shutdown closures.
This ensures WebSocket connections are safely disconnected when the
server shuts down. Additionally, APIs are updated to use pointer
arguments for consistency and ease of passing objects.

Details:
1. `EmbeddedTestServer` Enhancements:
   - Added `RegisterShutdownClosure` method to allow registering
     closures that are invoked before server shutdown completes.
   - `ShutdownOnIOThread` now calls all registered shutdown closures to
     ensure proper cleanup of WebSocket connections.

2. `WebSocketConnection` Enhancements:
   - Introduced a `shutdown_subscription_` member to manage the shutdown
     closure registration.
   - The constructor now accepts an `EmbeddedTestServer*` and registers
     a shutdown closure using `RegisterShutdownClosure`.

3. API Updates:
   - Changed `EmbeddedTestServer` arguments in relevant methods to
     pointers for consistency.
   - Updated `CreateWebSocketHandler`,
     `InstallDefaultWebSocketHandlers`, and related methods to use
     pointers instead of references.

4. Test Changes:
   - Adjusted tests and utility functions to pass `EmbeddedTestServer*`
     instead of references.

Reason for Change:
- To ensure safe cleanup of WebSocket connections during server
  shutdown, reducing the risk of dangling connections or resource leaks.
- Using pointer arguments simplifies API usage and aligns with Chromium
  coding conventions.

Bug: 40812029
Change-Id: I40b002435458b07e3415c8e2be70c4f55798bd37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6023255
Auto-Submit: Ryosuke Miyake <rysk@google.com>
Commit-Queue: Adam Rice <ricea@chromium.org>
Reviewed-by: Huanpo Lin <robertlin@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1385450}