Write out CacheStorageCache size to index file.

Persisting the size allows CacheStorage and CacheStorageManager to avoid
using simple cache to unnecessarily calculate the cache size if it is
already known.

BUG=617963

Review-Url: https://codereview.chromium.org/2416713002
Cr-Commit-Position: refs/heads/master@{#440425}
18 files changed