Merge SyncPrefs::SetKeepEverythingSynced and SetPreferredDataTypes

In practice (outside of tests), these two are always called together,
and setting them individually is a bit weird/misleading at best. So
let's merge them into one setter.

This will ease some coming refactorings (see bug), and it is also
more consistent with GetPreferredDataTypes, which respects the
sync_everything bit already.

Bug: 884159
Change-Id: I1b0fa0ccceb874c17703e1f4ceda7b895a5d7969
Reviewed-on: https://chromium-review.googlesource.com/c/1402784
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621163}
6 files changed