Ensure tests have an active task runner

chrome/browser/prefs/pref_model_associator_unittest.cc

Index: chrome/browser/prefs/pref_model_associator_unittest.cc
diff --git a/chrome/browser/prefs/pref_model_associator_unittest.cc b/chrome/browser/prefs/pref_model_associator_unittest.cc
index 44e557740412b6d171a8e9b508f2daba6ce42f01..3bdf8dc985436a5f5ef9d9064d7d39afd0efa98c 100644
--- a/chrome/browser/prefs/pref_model_associator_unittest.cc
+++ b/chrome/browser/prefs/pref_model_associator_unittest.cc
@@ -8,6 +8,7 @@
 #include "chrome/browser/prefs/pref_model_associator.h"
 #include "chrome/common/pref_names.h"
 #include "chrome/test/base/testing_profile.h"
+#include "content/public/test/test_browser_thread_bundle.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 class AbstractPreferenceMergeTest : public testing::Test {
@@ -45,6 +46,7 @@ class AbstractPreferenceMergeTest : public testing::Test {
     pref_service_->Set(pref_name.c_str(), *empty_value);
   }
 
+  content::TestBrowserThreadBundle thread_bundle_;
   TestingProfile profile_;
   PrefService* pref_service_;
 };