Storage manager: Show Google Drive offline files.

This CL adds following functions:
- Show the total size of Google Drive offline files.
- Show a confirmation dialog from which users can delete evictable offline files.

BUG=591958
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2070893003
Cr-Commit-Position: refs/heads/master@{#400575}
10 files changed