Actually two flags which play in tandem:

--enable-local-sync-backend and --local-sync-backend-dir

The second is respected only if the first one is specified.
Eventually those will be controlled by policy but the flags will allow
easier testing and potentially unlock the feature for advanced users.

For now sign-in is needed to kick the sync logic on but the data
is only stored locally when this flag is specified.

This CL is identical to https://codereview.chromium.org/2442453002/
with only a changed type of the kLoopbackServerBackendFilename
constant according to the size bot recommendation.

BUG=651411
TEST=unit_tests/manual.
TBR=pavely

Review-Url: https://codereview.chromium.org/2473613003
Cr-Commit-Position: refs/heads/master@{#429558}