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FakeServer/LoopbackServer is extended with logic that mimics what real
servers do, in order to verify what contributes to synced history.

This functionality is leveraged to add proper integration test coverage
for various relevant scenarios to history sync, starting with the
simplest, to more advanced including closed tabs.

This approach is expected to fix some flakes we've observed in recent
attempts to land tests, because it doesn't suffer from server-side
session entities potentially being overwritten by newly open tabs.

Bug: 882489,910947
Change-Id: I4a69fa080e3deacc8d33c64de17f964c34436ea3
Reviewed-on: https://chromium-review.googlesource.com/c/1356811
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613127}