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Implement encrypting and decrypting large blobs, and add a test for
updating blobs which exercises the new code path.

Bug: 1114875
Change-Id: Ief91f642fa5ccc34f7ef2d00c90467fbcb90bc5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415218
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Auto-Submit: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#808134}