Move assignment operator of both classes contained a memory leak.
Changed type of mMessage from mutable std::string *to mutable
std::unique_ptr<std::string>

BUG=angleproject:1264

Change-Id: I7d1419b2e9f385b36afebfd6371983be33ee9e61
Reviewed-on: https://chromium-review.googlesource.com/319520Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tryjob-Request: Tibor Ouden, den <tibordenouden@gmail.com>
Tested-by: Jamie Madill <jmadill@chromium.org>