Add runtime flag for variation sequences support

Bug: 40628044
Change-Id: I1376159937f7fa874033bc2172d7bd7f7f4b8b62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5424793


Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Munira Tursunova <moonira@google.com>
Cr-Commit-Position: refs/heads/main@{#1283063}