Remove RandomNumber and RandomNumberSeed.

The classes are just shells, since we delegate to CryptographicallyRandomNumber
under the hood. And amusingingly, the one caller of randomNumber() incurs an
int -> float -> int conversion when all it wants is a random int.

The only caller of raw OS random APIs is inside fastMalloc() and specific to the
Mac port. It uses the Mac-specific arc4random(), which self-initializes. So the
removal of RandomNumberSeed does not affect anything in Blink either.

Review URL: https://codereview.chromium.org/26608003

git-svn-id: svn://svn.chromium.org/blink/trunk@159191 bbb929c8-8fbe-4397-9dbb-9b2b20218538