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

base/metrics/stats_table_unittest.cc

Index: base/metrics/stats_table_unittest.cc
diff --git a/base/metrics/stats_table_unittest.cc b/base/metrics/stats_table_unittest.cc
index 0a1c38bbf0021c317575db390d1cf5ff2f58282c..2755aceba5bc4a3c8928a52111d2b7ef901a80d7 100644
--- a/base/metrics/stats_table_unittest.cc
+++ b/base/metrics/stats_table_unittest.cc
@@ -106,8 +106,7 @@ void StatsTableThread::Run() {
 }
 
 // Create a few threads and have them poke on their counters.
-// Flaky, http://crbug.com/10611.
-TEST_F(StatsTableTest, DISABLED_MultipleThreads) {
+TEST_F(StatsTableTest, MultipleThreads) {
   // Create a stats table.
   const std::string kTableName = "MultipleThreadStatTable";
   const int kMaxThreads = 20;