[ios] Make all Snackbars with actions use legacy dismissal behavior

Because something is wrong with the autoroller for MDC, the property
useLegacyDismissalBehavior had to be declared in a category in order to
access it. A further run-time check is added to ensure it is there. A
killswitch is also added for safety.

Fixed: 332491188
Change-Id: I1bb0e8768cf3e8b6e7d74c007ed2cd9680c0474e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5438456
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Scott Yoder <scottyoder@google.com>
Cr-Commit-Position: refs/heads/main@{#1284708}