The most notable change is in how the integrity of the tag cache file is
checked. Once deployed, existing tag cache files will be considered invalid and
wiped.

The only observable side effect is the following line in the log:
"can't deserialize tag cache - no sha256 is recorded in the file"

R=tandrii@chromium.org, iannucci@chromium.org

Change-Id: I7ea300a4e7ad8be0c1d42ae561c5202420d2db62
Reviewed-on: https://chromium-review.googlesource.com/c/1327823Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>