We were observing UI jank, which ended up being due to GC. Even with chunking
and very small chunk sizes, it was hard to reliably get 30fps on my macbook.
Moving to a worker completely solves the problem. We stay at 60fps the entire
time the hashing is taking place, no matter how many files/how large, etc.
Also, switch to using Closure's crypto support, which has been added since
the upload code was originally written, since it hassupport for incremental
hashing, javascript typed arrays, and other niceties.
Change-Id: I018d6839b2cf037b8d6b03e1a0ea7164bc5a782d
Aaron Boodman