Check that table evenly divides 256 when generating random string.

webrtc/base/helpers.h

Index: webrtc/base/helpers.h
diff --git a/webrtc/base/helpers.h b/webrtc/base/helpers.h
index c75dba59854184c0e9ed7cfd0d38a631610c3f90..11e5e87432b8fa133de02f3e8992fadd79b95d6f 100644
--- a/webrtc/base/helpers.h
+++ b/webrtc/base/helpers.h
@@ -36,6 +36,8 @@ bool CreateRandomString(size_t length, std::string* str);
 // Generates a (cryptographically) random string of the given length,
 // with characters from the given table. Return false if the random
 // number generator failed.
+// For ease of implementation, the function requires that the table
+// size evenly divide 256; otherwise, it returns false.
 bool CreateRandomString(size_t length, const std::string& table,
                         std::string* str);