Run NonThreadSafeDeathTests in the threadsafe gtest mode.

On Mac, //base/threading uses CoreFoundation, which causes forked child
processes to violate the framework's requirement that CF may not be used
after-fork-before-exec.

BUG=717490
R=mark@chromium.org

Review-Url: https://codereview.chromium.org/2865453002
Cr-Commit-Position: refs/heads/master@{#469375}

base/threading/non_thread_safe_unittest.cc

@@ -113,6 +113,7 @@ void NonThreadSafeClass::MethodOnDifferentThreadImpl() {
 
 #if DCHECK_IS_ON()
 TEST(NonThreadSafeDeathTest, MethodNotAllowedOnDifferentThreadInDebug) {
+  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
   ASSERT_DCHECK_DEATH({ NonThreadSafeClass::MethodOnDifferentThreadImpl(); });
 }
 #else
@@ -136,6 +137,7 @@ void NonThreadSafeClass::DestructorOnDifferentThreadImpl() {
 
 #if DCHECK_IS_ON()
 TEST(NonThreadSafeDeathTest, DestructorNotAllowedOnDifferentThreadInDebug) {
+  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
   ASSERT_DCHECK_DEATH(
       { NonThreadSafeClass::DestructorOnDifferentThreadImpl(); });
 }