[Storage] Blob items are now shared between blobs. Ready for disk swap.
  * Added ShareableBlobDataItem for sharing items between blobs, and
    implemented blob item sharing.
  * Added InternalBlobData for the internal storage of blobs.
  * Removed CreateSnapshot ability from the old Builder, which is now
    exclusively used to tell the context to create a blob.
  * Fixed copying of the content_disposition header when
    programmatically creating a blob.
  * Fixed UMA histogram for files that have uint64max length value
    (which signifies 'end-of-file' without knowing the size), and
    added a histogram to track how often this happens.
  * Added more documentation and histograms for knowing the state of
    blobs on the web.

Design Doc: https://bit.ly/AutoBlobToDisk

R=michaeln
BUG=375297

Review URL: https://codereview.chromium.org/895933007

Cr-Commit-Position: refs/heads/master@{#315702}
27 files changed