The option to "enable serialization" actually only enables the
roots-relative register addressing. In the spirit of expressing specific
behaviors rather than isolate-wide modes, rename this flag to what
it actually does.

R=bmeurer@chromium.org

Change-Id: Iacbad8064c8f38b364db2a0bdd0e83ef4a265867
Reviewed-on: https://chromium-review.googlesource.com/1113742Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54016}