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When the "Sync to another account" button is clicked in the DICE
user menu, a submenu showing a list of web accounts (other than the
first one) should be displayed. This CL adds this feature.

Mocks can be found in the bug report.

Bug: 786369
Change-Id: I32b4f3f19472a67e7aa203426692739030acc4c9
Reviewed-on: https://chromium-review.googlesource.com/893570
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534036}