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This change makes PlatformKeysServiceImpl independent of
BrowserContext/Profile.
All BrowserContext-specific calls have been extracted into a
PlatformKeysServiceImplDelegate.

This has the advantage that for device-wide usage of the
PlatformKeysService, a delegate can be created which does not depend on
a Profile (avoiding using the sign-in Profile for that use case).
The CertProvisioningScheduler use case has been migrated to this
pattern.

As the device-wide PlatformKeysService lives "forever" but the
underlying system token NSSCertDatabase does not, PlatformKeysService
can now explicitly "shut down" when the underlying NSSCertDatabase is
being destroyed. After it is shut down all operations on the
PlatformKeysService will fail with an error (except for removing
observers).

Note: The PlatformKeysService still works for the sign-in Profile
which is important for the sign-in screen extensions use case.

Bug: 1045895, 1073512
Test: browser_tests
Change-Id: I09b855c61caaa744bf8618073eaa2854a609df84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2188633
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Michael Ershov <miersh@google.com>
Cr-Commit-Position: refs/heads/master@{#790397}