GitLab

- Refactored VAPID key manager to separate get key and refresh key
- Derive from sync secret if kSharingDeriveVapidKey is enabled
- Observe for VAPID key source changes, refresh cached key and
  re-register if cached VAPID key has changed

Bug: 1010968
Change-Id: I178a6cd931ad41ab0184dd8a511e8ac0569e2269
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848171
Commit-Queue: Alex Chau <alexchau@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706399}