Enable ServiceWorkerBackgroundUpdateForRegisteredStorageKeys on WebView

This CL
- renames BackgroundUpdateForRegisteredStorageKeys (introduced in
  crrev.com/c/6606856 to
  ServiceWorkerBackgroundUpdateForRegisteredStorageKeys
- enables it on WebView prior to other platforms in order to improve
  crbug.com/418159642. For other platform, it is still disabled by
  default.
- adds another flag
  ServiceWorkerBackgroundUpdateForRegisteredStorageKeysFinchControlled
  because on WebView the original flag may be overridden as above and
  will not be controllable from finch.

Bug: 421530699, 418159642
Change-Id: I7ae596e065cfd262963421855a84f79098879518
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6614885
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Taiyo Mizuhashi <taiyo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1470369}