Add ClientSharedImage::MakeUnowned()

In the ClientSharedImage refactoring, the direct replacement of
MailboxHolder with ClientSharedImage in some shared image clients could
cause DestroySharedImage() to fail since the ClientSharedImage may
have more than one scoped_refptr reference at the destruction time.
As a workaround, this CL adds a ClientSharedImage::MakeUnowned()
so that clients can create unowned ClientSharedImage copies instead of
adding references to the original ClientSharedImage.

Bug: 1494911
Change-Id: Ic3db225fe39396c93198e984a5682156b7433e4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5385546
Commit-Queue: Mingjing Zhang <mjzhang@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1276931}