[rng] improve RNG seed.

Prior to this change, both 0 and 1 as RNG seed would result in the same
internal state. state0 and state1 cannot both be zero, but murmur hash
maps 0 back to 0.

R=cbruni@chromium.org
BUG=v8:5069

Review-Url: https://codereview.chromium.org/2040953002
Cr-Commit-Position: refs/heads/master@{#36757}

src/base/utils/random-number-generator.cc

@@ -124,10 +124,10 @@ int RandomNumberGenerator::Next(int bits) {
 
 
 void RandomNumberGenerator::SetSeed(int64_t seed) {
-  if (seed == 0) seed = 1;
   initial_seed_ = seed;
   state0_ = MurmurHash3(bit_cast<uint64_t>(seed));
-  state1_ = MurmurHash3(state0_);
+  state1_ = MurmurHash3(~state0_);
+  CHECK(state0_ != 0 || state1_ != 0);
 }