Consolidate clock sources to OfflineClock()

This change further changes direct calls to Time::Now or the default
base::Clock instance to an OfflineClock module for ease of testing
within production files and their related tests in:

components/offline_pages/core/background/

Bug: 906903
Change-Id: Iee19708b9297e3e3b3944c457b163d5555914948
Reviewed-on: https://chromium-review.googlesource.com/c/1352045
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Mark Lieu <mtlieuu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612293}
19 files changed