Use TimeTicks instead of Time for measuring elapsed time

TimeTicks is preferred for measuring time, when time isn't being measured
across restarts. These were the only two cases I found under
components/offline_pages that could be converted to TimeTicks.

Change-Id: I627698de121cdda5d7853e0d92f7032ade524a40
Reviewed-on: https://chromium-review.googlesource.com/c/1364080
Commit-Queue: Dan H <harringtond@google.com>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615604}
4 files changed