guest-contents: attach guest contents with frame token

GuestContentsHost is a lazy binding interface optionally provided by a
WebUIController. The renderer uses this interface to replace the inner
document of a frame with a guest contents identified by guest id.

On the renderer side, this CL adds a helper function SwapRenderFrame()
that leverages GuestContentsHost to swap a frame with a inner
WebContents.

This CL demonstrates the registration of GuestContentsHost in
webui_examples. The follow up CL will migrate webui_examples to
guest-contents and actually use the interface.

Bug: 415626990
Validate-Test-Flakiness: skip
Change-Id: I4136eb4fddb20b7c6a44ceb46e79c1154d6c4d40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6612221
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1470749}