Right now we hold on to all the memory until the browser says it's done building the blob.

For large blobs, we can get rid of the memory during the shared memory or file reading / writing, freeing up much-needed space for it to live in the browser.

See BlobConsolidation and BlobTransportController. 

Main issue is that we proactively read for the shortcut IPC path, so make sure we don't get rid of memory that time.