Basic implementation of key fetcher for Phase 1a

This CL only implements the functionality of requesting a key from
storage and checking its expiry. Other functionalities, e.g. fetching
keys from helper servers on first use and when the chosen key has
expired, using the data decoder service's JSON parser to parse the keys,
storing parsed keys to storage, will be addressed in a future CL in
Phase 1b.

Bug: 1227145
Change-Id: I9657f7eacd37ffd1b0569d52d3a1e6d5f160150d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2979893
Commit-Queue: Nan Lin <linnan@chromium.org>
Reviewed-by: John Delaney <johnidel@chromium.org>
Reviewed-by: Alex Turner <alexmt@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#902703}

• content/browser/BUILD.gn[diff]
• content/browser/aggregation_service/aggregation_service_key_fetcher.cc[Added - diff]
• content/browser/aggregation_service/aggregation_service_key_fetcher.h[Added - diff]
• content/browser/aggregation_service/aggregation_service_key_fetcher_unittest.cc[Added - diff]
• content/browser/aggregation_service/aggregation_service_test_utils.cc[diff]
• content/browser/aggregation_service/aggregation_service_test_utils.h[diff]
• content/browser/aggregation_service/public_key.cc[diff]
• content/browser/aggregation_service/public_key.h[diff]
• content/test/BUILD.gn[diff]
• content/test/OWNERS[diff]
• content/test/test_aggregation_service_impl_unittest.cc[diff]