IPC fuzzer: move from /dev/urandom to a PRNG.

Functions in base/rand_util.h read from /dev/urandom. Each function call does a system call.
Mutating an ipcdump with 3000 messages does around 1000000 reads from /dev/urandom, which takes about a second on my machine. Moving to a PRNG instead.

APP_PATH=... time tools/ipc_fuzzer/mutate/ipc_fuzzer_mut.py --input_dir=... --output_dir=... --no_of_files=10

Previously:
real    0m13.262s
user    0m2.550s
sys 0m10.000s

Now, 79% faster:
real    0m2.763s
user    0m1.590s
sys 0m0.630s

BUG=260848

Review URL: https://codereview.chromium.org/145133011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246734 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246734