GitLab

Offline pages metadata store:
* Creates interface for the store
* The following actions are available on the store:
  - AddOfflinePage,
  - RemoveOfflinePage,
  - Load,
* Updates to the components_unittests to include offline
  pages

BUG=491352

Review URL: https://codereview.chromium.org/1160283003

Cr-Commit-Position: refs/heads/master@{#333697}