[ios][web] Prepare RealizedWebState API to split data and metadata

Change RealizedWebState constructor to take the creation time. This
allow marking some member fields of RealizedWebState as const since
they are never changed during the lifetime of the object.

Change RealizedWebState::Init() method to receive metadata from the
SerializedData object (will be required when data and metadata are
stored in separate files).

Bug: 1383087
Change-Id: Ifdd9055514d0a964a6d7193cc8f88f48cf85df55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4670895
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Federica Germinario <fedegermi@google.com>
Cr-Commit-Position: refs/heads/main@{#1168044}