Improve SharedMemory::Lock on Posix and reenable StatsTableTest.MultipleThreads

base/shared_memory_posix.cc

Index: base/shared_memory_posix.cc
diff --git a/base/shared_memory_posix.cc b/base/shared_memory_posix.cc
index 32a55e91284f21d5abc299e376dd81192f668d4e..575089b6b69c91783267bbcaf667de42d49ff5cf 100644
--- a/base/shared_memory_posix.cc
+++ b/base/shared_memory_posix.cc
@@ -11,9 +11,11 @@
 #include <unistd.h>
 
 #include "base/file_util.h"
+#include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/threading/platform_thread.h"
 #include "base/safe_strerror_posix.h"
+#include "base/synchronization/lock.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/utf_string_conversions.h"
 
@@ -34,6 +36,8 @@ namespace {
 // namespaces, but who knows what's out there.
 const char kSemaphoreSuffix[] = "-sem";
 
+LazyInstance<Lock>::Leaky g_thread_lock_ = LAZY_INSTANCE_INITIALIZER;
+
 }
 
 SharedMemory::SharedMemory()
@@ -268,11 +272,13 @@ void SharedMemory::Close() {
 }
 
 void SharedMemory::Lock() {
+  g_thread_lock_.Get().Acquire();
   LockOrUnlockCommon(F_LOCK);
 }
 
 void SharedMemory::Unlock() {
   LockOrUnlockCommon(F_ULOCK);
+  g_thread_lock_.Get().Release();
 }
 
 #if !defined(OS_ANDROID)