[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