Preparation to convert GetPrimaryAccount to return CoreAccountInfo

Change client code to work as if IdentityManager::GetPrimaryAccount
where returning a CoreAccountInfo instead of an AccountInfo. Method
will be updated in a followup CL.

This CL was uploaded by git cl split.

Bug: 926204
Change-Id: Ib4d459b087da9e02deeb127bb17166a64e99a686
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1499562
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641577}