[LibFuzzer] Fix `FuzzerMutate.ShuffleBytes2` unit test on OSX.

[delcypher]

Summary

[LibFuzzer] Fix FuzzerMutate.ShuffleBytes2 unit test on OSX.

The FuzzerMutate.ShuffleBytes2 unit test was failing on
OSX due to the implementation of std::random_shuffle()
being different between libcxx and libstdc++.

@kcc has decided (see http://reviews.llvm.org/D21218) it is acceptable
for there to be different mutation behavior on different platforms so
this commit just adjusts the test to perform the minimum number of
iterations (that is a power of 2) to see all the mutations the unit test
is looking for.

[kcc]

LGTM

lib/Fuzzer/test/FuzzerUnittest.cpp

262

the comment is not very useful, please remove it.
The commit message will be enough.

[delcypher]

Remove comment.

lib/Fuzzer/test/FuzzerUnittest.cpp

void TestShuffleBytes(Mutator M, int NumIter) {
   }
   EXPECT_EQ(FoundMask, 31);
 }
 
 TEST(FuzzerMutate, ShuffleBytes1) {
   TestShuffleBytes(&MutationDispatcher::Mutate_ShuffleBytes, 1 << 16);
 }
 TEST(FuzzerMutate, ShuffleBytes2) {
-  TestShuffleBytes(&MutationDispatcher::Mutate, 1 << 19);
+  TestShuffleBytes(&MutationDispatcher::Mutate, 1 << 20);
 }
 
 void TestAddWordFromDictionary(Mutator M, int NumIter) {
   std::unique_ptr<ExternalFunctions> t(new ExternalFunctions());
   fuzzer::EF = t.get();
   Random Rand(0);
   MutationDispatcher MD(Rand);
   uint8_t Word1[4] = {0xAA, 0xBB, 0xCC, 0xDD};