Replace usage of md5 with sha256 for generation of mojo app ids.

Replace usage of md5 with sha256 for generation of mojo app ids.

Before we start adding more uses of mojo app ids we should use
to a non-broken hash function. :)

What was holding me back before was I wasn't aware of us having an
incremental hash api other than base/md5.h (others in base only operate
on the full input data), however it turns out that the crypto/ library
has one in crypto/secure_hash.h.

R=abarth@chromium.org
BUG=

Committed: https://chromium.googlesource.com/external/mojo/+/ca9659ccbfe3311a40970ae4aa11e8364e3257b3

[abarth-chromium, 2015-01-29 20:59:14]

lgtm

[eseidel, 2015-01-29 20:59:54]

Committed patchset #1 (id:1) manually as
ca9659ccbfe3311a40970ae4aa11e8364e3257b3 (presubmit successful).

[qsr, 2015-01-30 15:15:58]

qsr@chromium.org changed reviewers:
+ qsr@chromium.org

[qsr, 2015-01-30 15:15:59]

https://codereview.chromium.org/868253006/diff/1/shell/dynamic_application_lo...
File shell/dynamic_application_loader.cc (right):

https://codereview.chromium.org/868253006/diff/1/shell/dynamic_application_lo...
shell/dynamic_application_loader.cc:317: // Currently we use sha256 from
crypto/secure_hash.h
Eric: there is a few things to consider for this:
1) Only hash a few k of the library, that should be enough to get a hash, and
will be quicker to compute and need less I/O.
2) Try to compute the has while we get the byte from the network in case of a
network load, so that we do not have to re-read the same bytes
3) Fix the current race -> if the same application is in 2 different location on
the network, and we load those at the same time, there is a race.