[iOS] Fix bug in side-swipe snapshot for overlays.

Side-swiping between web views captures a snapshot synchronously prior
to displaying the transition between tabs side by side. However, because
the snapshotting action is asynchronous the snapshot may not have been
captured prior to display. This is most obvious in overlays since the
snapshot is captured on page loaded and the overlay is added by a user
action after this.

Bug: 417956837
Change-Id: Iffeed00f309f8017229b1caef7c16f3282ce80d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6578390
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Asami Doi <asamidoi@chromium.org>
Commit-Queue: Quentin Pubert <qpubert@google.com>
Cr-Commit-Position: refs/heads/main@{#1465418}