Adding a previews IO-thread blacklist

components/previews/previews_experiments_unittest.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/previews/previews_experiments.h"
 
 #include "base/metrics/field_trial.h"
+#include "components/variations/variations_associated_data.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace {
 
 using PreviewsExperimentsTest = testing::Test;
 
 }  // namespace
 
 namespace previews {
 
 TEST_F(PreviewsExperimentsTest, TestFieldTrialOfflinePage) {
   EXPECT_FALSE(IsIncludedInClientSidePreviewsExperimentsFieldTrial());
   EXPECT_FALSE(IsOfflinePreviewsEnabled());
 
   base::FieldTrialList field_trial_list(nullptr);
   ASSERT_TRUE(EnableOfflinePreviewsForTesting());
 
   EXPECT_TRUE(IsIncludedInClientSidePreviewsExperimentsFieldTrial());
   EXPECT_TRUE(IsOfflinePreviewsEnabled());
+  variations::testing::ClearAllVariationParams();
 }
 
 }  // namespace previews