WebXR: Propagate multiple shared images for Layers API

The WebXR Layers API requires the ability to handle multiple shared
images in the onFrame callback. This change introduces the concept of an
`XRSharedImageSource` to manage shared images from the base layer,
camera, and API layers in a unified way.

This approach allows us to consolidate shared images returned via mojom
into a single array. These images can originate from: 1. The Base Layer
(which does not have an associated layer ID on the backend). 2. The
Camera Shared Images. 3. WebXR Layers API layers (Projection, Quad,
Cylinder, Equirect, Cube).

This change provides the initial infrastructure to support multiple
shared images, unblocking further WebXR Layers API implementation.

Bug: 441138904
Change-Id: Id2b6a257b9f4dcc941414433f98b6c7385ce56e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6885041
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Evgenii Alekseev (xWF) <ealekseev@google.com>
Cr-Commit-Position: refs/heads/main@{#1513349}