No real changes, just avoids doling out Uint64 sized chunks by
using the underlying method to hand out correctly sized blocks.

Windows is the only platform which doesn't have a byte stream
based generator, so I've moved the generic RandBytes() method
there and added native methods for other platforms which reuse
each platforms internal generator.

Performance measured by the new benchmark test over 5 runs,
each representing 10 generations of 1mb of random data:

Linux x64:
Original: 1199625.4
Modified: 686480.2
Improvement: 1.75x

On OSX (10.9.1):
Original: 1532669.8
Modified: 734808.0
Improvement: 2.10x

BUG=none
TEST=new benchmark unittest.

Review URL: https://codereview.chromium.org/140773006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246486 0039d316-1c4b-4281-b951-d872f2087c98