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This CL adds access token fetching functionality, that can be used on
trusted vault backend sequence. Implementation consists of:

TrustedVaultAccessTokenFetcherFrontend:
 * this class must be used on the UI thread
 * it allows asynchronous access token fetching
 * it supports multiple ongoing FetchAccessToken() calls (e.g. the
 method can be called before |callback| passed to the previous one was
 run)
 * it observes primary account changes and reply with empty access token
 to callbacks which correspond to non-primary account

TrustedVaultAccessTokenFetcherImpl:
 * created on the UI thread and its ownership transferred to the backend
 sequence
 * plumbs backend FetchAccessToken() attempts to the frontend
 * handles invalidation of TrustedVaultAccessTokenFetcherFrontend
 WeakPtr

Bug: 1113597
Change-Id: I72d5962c374b3141d8a20a2ad2a4b42a378bbfde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2379734
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802650}