Make HistoryDataStoreTest and SearchHistoryTest's SequencedWorkerPool multi-threaded.

BUG=646443

Review-Url: https://codereview.chromium.org/2345583002
Cr-Commit-Position: refs/heads/master@{#419085}

chrome/browser/ui/app_list/search/history_unittest.cc

@@ -96,7 +96,7 @@ class SearchHistoryTest : public testing::Test {
   // testing::Test overrides:
   void SetUp() override {
     worker_pool_owner_.reset(
-        new base::SequencedWorkerPoolOwner(1, "AppLauncherTest"));
+        new base::SequencedWorkerPoolOwner(2, "AppLauncherTest"));
     ASSERT_TRUE(temp_dir_.CreateUniqueTempDir());
     CreateHistory();
   }

ui/app_list/search/history_data_store_unittest.cc

@@ -41,7 +41,7 @@ std::string GetDataContent(const HistoryData::Data& data) {
 
 class HistoryDataStoreTest : public testing::Test {
  public:
-  HistoryDataStoreTest() : worker_pool_owner_(1, "AppLanucherTest") {}
+  HistoryDataStoreTest() : worker_pool_owner_(2, "AppLanucherTest") {}
 
   // testing::Test overrides:
   void SetUp() override {