Changes from last review:
Added support for mapping a tab that had already been mapped to a different
window. This can happen in Android when a placeholder tab is created from
a previous window that hasn't yet closed. In addition, for testing/repro
purposes this patch makes the iteration order of windows deterministic (
by going through them in SessionID order).

Previous review: https://codereview.chromium.org/2683263002/

TEST=SessionsSyncMagerTest.PlaceholderConflictAcrossWindows
BUG=639009

Review-Url: https://codereview.chromium.org/2712743006
Cr-Commit-Position: refs/heads/master@{#453992}