[shared storage] Move the implementation of sharedStorage.selectURL()/run() to the SharedStorageWorklet class

This prepares for a potential future change to actually expose
selectURL()/run() on the sharedStorage.worklet JS object.

A non-web-visible behavior difference: when selectURL()/run() is called
without addModule(), we still construct `shared_storage_worklet_`, even
though we could fail it early. This is because we prefer a cleaner code
over saving the overhead of constructing `shared_storage_worklet_`.

Bug: 1218540
Change-Id: I47ad32caa6b381a122909b3d9d08c9547d0d6281
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4955983
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Reviewed-by: Cammie Smith Barnes <cammie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1212930}