GitLab

Added some utility functions for cached offline pages:
Get sizes of all cached offline pages;
Delete all cached offline pages.
Also added the usage of cached offline pages to the number shown at:
Chrome Settings -> Privacy -> Clear Browsing Data -> Cache

BUG=716142

Review-Url: https://codereview.chromium.org/2860573004
Cr-Commit-Position: refs/heads/master@{#472255}