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This CL augments the existing ServiceManager embedding on iOS with
support for per-user embedded services. A "per-user" service is one
that has one instance per user, rather than a shared instance for all
users. Concretely, each BrowserState has a unique UserID, and there is
one instance of a per-user service per UserID. Essentially, one can
think of per-user services as the equivalent of profile-keyed services
in the services world.

The support added in this CL closely follows that of //content:

- Public APIs are identical
- Implementation in BrowserState follows that of BrowserContext with minor
  changes:
  - iOS doesn't use File Service at this time
  - No client of //ios/web needs to override how a user ID is generated

This CL adds an Earl Grey test of web_shell that a per-user service can be
embedded and if connected to from a given BrowserState, has its user ID
properly associated with that of the BrowserState.

Finally, this added support requires that all BrowserState subclasses call
BrowserState::Initialize(). This is also done in this CL.

Bug: 731588
Change-Id: Ie3bb79d4f754aecddd9c35c72576262dfc87cb55
Reviewed-on: https://chromium-review.googlesource.com/559529Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485547}