Record DRP histogram on how long back savings were cleared

Record Data Reduction Proxy (DRP) histogram on how long back savings were
automatically cleared because the system clock moved back. If this happens
too frequently, then we will need to add a mechanism to get around this.
This would also help in debugging 0% savings feedback reports.

BUG=658354

Review-Url: https://codereview.chromium.org/2546273002
Cr-Commit-Position: refs/heads/master@{#438011}
9 files changed