Create target in //components/signin for shared code

//components/signin/core/browser contains three different types of code:

1. Code that forms the guts of IdentityManager and will form the guts of
the Identity Service implementation (e.g., SigninManager).

2. Code that will be clients of IdentityManager (e.g., SigninTracker).

3. Code that will be used on both sides (e.g., AccountInfo).

1 and 2 have already been split via a preceding CL. This CL creates 3.
To start off with, 3 contains two pieces of code that were already in
their own standalone targets for similar motivations as the above:
account_info.* and signin_metrics.*. In the future more code will
likely move in.

Bug: 901859
Change-Id: Ibcae3c9f1ecff2dc7ccd013046b74c97b3dd2b7a
Reviewed-on: https://chromium-review.googlesource.com/c/1323114
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606783}
7 files changed