ServiceWorker: Remove the runtime flag for updateViaCache

This flag is no longer necessary because updateViaCache was enabled by default
in M68:
https://www.chromestatus.com/feature/6059838387781632

Bug: 675540
Change-Id: I03c5e00b83b150f83edd7022dd38d1fa657d324f
TBR: kinuko@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1249471
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594984}
3 files changed