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This CL add histogram Signin.SignedInAccountsViewImpression that
counts the number of times the signed-in account view is shown when
the application enters foreground (iOS only).

Bug: 1018211
Change-Id: Iff7f8799a8ecb0b13858d3cb3b744738c01adae7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2106170
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753202}