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The feature has been running for a while on 50% canary and dev
with only one known issue which is already fixed on trunk.

Hence, let's enable it by default, which also requires updating
a few tests.

The switch exercises a more modern implementation of sessions
sync that should be identical from the users perspective, besides
the performance improvements (most notably, memory consumption
estimated to be reduced by a factor of 70% according to metric
Sync.ModelTypeMemoryKB.SESSION), or ~350 KB for the 95th
percentile.

Bug: 681921
Change-Id: Ida3e676a534d8c3ced174092d5498a793860020b
Reviewed-on: https://chromium-review.googlesource.com/1086945
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568045}