Fix flakyness in PrefMemberTest::MoveToThread

base/prefs/public/pref_member_unittest.cc

Index: base/prefs/public/pref_member_unittest.cc
diff --git a/base/prefs/public/pref_member_unittest.cc b/base/prefs/public/pref_member_unittest.cc
index 4203db3aa3caabe01bb771d9897cf976f472378c..eab5a4cc986032c55012c54ff62a2e19fd49c80a 100644
--- a/base/prefs/public/pref_member_unittest.cc
+++ b/base/prefs/public/pref_member_unittest.cc
@@ -56,6 +56,12 @@ class GetPrefValueHelper
     event.Wait();
   }
 
+  // The thread must be stopped on the main thread. GetPrefValueHelper being
+  // ref-counted, the destructor can be called from any thread.
+  void StopThread() {
+    pref_thread_.Stop();
+  }
+
   bool value() { return value_; }
 
  private:
@@ -313,4 +319,6 @@ TEST(PrefMemberTest, MoveToThread) {
 
   helper->FetchValue();
   EXPECT_TRUE(helper->value());
+
+  helper->StopThread();
 }