[DOM Storage] Replace string source with StorageAreaSource struct

Replace the opaque string `source` parameter in the StorageArea mojom
interface and StorageAreaObserver with a new StorageAreaSource struct
containing typed `page_url` (url.mojom.Url) and `storage_area_id` (mojo_base.mojom.Token, mapped to base::Token)  fields.

Previously, the page URL and storage area ID were packed into a single
newline-delimited string. This was fragile and required manual
pack/unpack helpers (PackSource/UnpackSource). The new struct makes the
API self-documenting and type-safe.


Bug: 477645291
Change-Id: I3cb2a79b41740bad0ca9f8fa60fad807f69c408b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7597837
Commit-Queue: Xiaohan Zhao <xiaohanzhao@microsoft.com>
Reviewed-by: Steve Becker <stevebe@microsoft.com>
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1600287}