Variables of type {ExecutionTier} should be called "tier", not "mode".

R=ahaas@chromium.org

Bug: v8:8238
Change-Id: I09c640398ab8ad34ac6591d42ea7b0f9ba83d620
Reviewed-on: https://chromium-review.googlesource.com/c/1335688Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57502}