Fix memory leak in logging.cc

The change in my CL solves the memory leak problem, but introduces a data race in the following function of logging.cc:
Mutex *InitLogSilencerMutex() {
    static Mutex *m = new Mutex();  // Without C++11, this statement has a data race.
    return m;
}

The memory leak reported by tcmalloc:
Leak of 336 bytes in 6 objects allocated from:
        @ 4c6065 internal::InitLogSilencerCountOnce
        @ 4c5e8e internal::LogMessage::Finish
        @ 495523 orion::LmctfyContainerManager::Destroy
        @ 40fa39 DestroyExisting
        @ 40f95f TearDown
        @ 7b46b6 testing::internal::HandleSehExceptionsInMethodIfSupported
        @ 7b056e testing::internal::HandleExceptionsInMethodIfSupported
        @ 798ff6 testing::Test::Run
        @ 799716 testing::TestInfo::Run
        @ 799d32 testing::TestCase::Run
        @ 7a03f6 testing::internal::UnitTestImpl::RunAllTests
        @ 7b53b3 testing::internal::HandleSehExceptionsInMethodIfSupported
        @ 7b120e testing::int