Fix flaky FeedStoreTest.OverwriteStream and OverwriteStreamWebFeed failure.

The tests expects kTestTimeEpoch == base::Time::UnixEpoch().
Since FeedApiTest::SetUp() set kTestTimeEpoch to be Now(), FeedApiTest::TearDown() must reset kTestTimeEpoch to be base::Time::UnixEpoch().

Change-Id: Icbb942e454601f71d3c37c0e2f19eb83436654b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2777954
Commit-Queue: Dan H <harringtond@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#867668}