Use RtlGenRandom() directly instead of going through rand_s().

Results in a massive 44x improvement to RandBytes() for larger
buffer sizes.  Using the RandBytesPerf test, the results are:

Original: 2961196.6us
Modified: 63656.8us
Improvement: 44x

BUG=none
TEST=randbytes unittests.
NOTRY=true

Review URL: https://codereview.chromium.org/141753009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246736 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed