Perf improvements for sha1.js and sha2.js

allocating additional memory (and incurring additional garbage
collections) unnecessarily. ~10% speed gain on Chrome 35.

goog/crypt/sha2.js: Port the block-size fast path from Sha1.update
to Sha2.update. Compute message schedule on the fly. Remove code
for testing for invalid inputs that is not present in Sha1.update.
~30% speed gain on Chrome 35.

goog/crypt/sha256_test.html: Remove testBadInput(); no similar test
is performed in sha1_test.html
computeChunk_ takes no arguments.  It assumes chunk is a block-sized array containing the message chunk to update.  With this change this.chunk_ doesn't appear to be updated when iterating over the full blocks.  Amazingly the tests still pass(!).