Improve base::RandBytes() performance by 1.75x-2.10x on POSIX.

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
Let Chrome Elf know when Chrome has started sooner

In the old location Chrome could have exited (safely) before Chrome Elf
was alerted that Chrome was working correctly. This was causing
successful blacklist setups to get counted as failures.

BUG=

Review URL: https://codereview.chromium.org/143823013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246487 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246487