[ios] Use SnapshotBrowserAgent in CardSideSwipeView

When the CreateTabHelperOnlyForRealizedWebStates feature is enabled,
the tab helpers won't be created for unrealized WebState. Change
CardSideSwipeView to use SnapshotBrowserAgent to retrieve the tabs'
snapshots (since this always work, for both realized and unrealized
WebStates).

Also change the implementation to leave the conversion from color
to greyscale to SnapshotBrowserAgent instead of re-implementing the
conversion in CardSideSwipeView.

Fixed: 440321349
Bug: 409299519
Change-Id: I0b959aa89c87308b446c7fecb8424c2fd78e8964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6878966
Reviewed-by: Asami Doi <asamidoi@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1505816}