sql: Add precondition checks to sql::Connection.

This CL adds DCHECKs covering a few of the preconditions in
sql::Connection's API and removes sql::TimeSource, which duplicates
base::TickClock.

Bug: none
Change-Id: Ia62874ccf339cf2338e388a45a41d789e3ef89e3
Reviewed-on: https://chromium-review.googlesource.com/1143132
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576589}
4 files changed