[BackButton] Update fieldtrials param name

Bug: 505134325
Change-Id: I8cfd5bfce568b0c46f804897bbd7bf3003ed1414
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7833567
Commit-Queue: Charles Hager <clhager@google.com>
Auto-Submit: Joel Tan-Aristy <jtanaristy@google.com>
Reviewed-by: Charles Hager <clhager@google.com>
Cr-Commit-Position: refs/heads/main@{#1627987}
NOKEYCHECK=True
GitOrigin-RevId: f8375e83371d4003d5c71909b9f071c716d1f382