AccountTrackerService needs to manually enable Network Fetches.
AccountTrackerService, when instantiated during Profile startup, can attempt network calls before all services all ready. This is prevented by created a specific EnableNetworkFetches() method which is called by the Profile once KeyedServices are initialized.
This is a re-land of https://codereview.chromium.org/
805883002/ (which was reverted) with the addition that on Android the AccountTrackerService still only executes a fetch if there's no gaia-id.
BUG=442011, 441399, 432286
Review URL: https://codereview.chromium.org/
812413002
Cr-Commit-Position: refs/heads/master@{#310629}