Move crypto_helpers from sync to base

base/rand_util.h

Index: base/rand_util.h
diff --git a/base/rand_util.h b/base/rand_util.h
index 4e902da2f356ea666098ca3d34b73518f418833a..541fd775ad3589c4c663374a80053a28545f2049 100644
--- a/base/rand_util.h
+++ b/base/rand_util.h
@@ -29,7 +29,14 @@ BASE_API uint64 RandGenerator(uint64 max);
 // Returns a random double in range [0, 1). Thread-safe.
 BASE_API double RandDouble();
 
-// Returns a random string of the specified length.
+// Fills |output_length| bytes of |output| with cryptographically strong random
+// data.
+BASE_API void RandBytes(void* output, size_t output_length);
+
+// Fills a string of length |length| with with cryptographically strong random
+// data and returns it.
+//
+// Not that this is a variation of |RandBytes| with a different return type.
 BASE_API std::string RandBytesAsString(size_t length);
 
 }  // namespace base