Make HistoryDataStoreTest and SearchHistoryTest's SequencedWorkerPool multi-threaded.

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

Index: chrome/browser/ui/app_list/search/history_unittest.cc
diff --git a/chrome/browser/ui/app_list/search/history_unittest.cc b/chrome/browser/ui/app_list/search/history_unittest.cc
index 23ced10c2ac9b4db9a3f50aca3413a3b6d67d052..350bac86279eb4c40f1692a3b414fdec9fb9a220 100644
--- a/chrome/browser/ui/app_list/search/history_unittest.cc
+++ b/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"));

[Matt Giuca, 2016/09/15 23:54:20]

I'm not really familiar with this code. Is there a good reason it has to be
multi-threaded in a unit test?

It is probably not going to change anything, but it means there's a new
possibility of flakiness (things happening out of order) that weren't flaky
before.

[gab, 2016/09/16 00:54:41]

In this test it's used to pass to a DictionaryDataStore
(https://cs.chromium.org/chromium/src/ui/app_list/search/dictionary_data_store...)
which in production receives a real (multi-threaded) SequencedWorkerPool. So if
this results in flakes it would mean DictionaryDataStore doesn't probably use a
SequencedWorkerPool.

i.e. if anything this brings the test closer to what runs in production.

Making the SWP multi-threaded (i.e. max_threads > 1) just means it's allowed to
have multiple threads, but if the caller only uses one sequence things are still
going to be sequenced on a single thread -- if that's the case this change is a
no-op in practice (but we want to do it regardless as we're looking to disallow
single-threaded SequencedWorkerPools so that assuming thread-affinity from a SWP
is never okay, users that want that should use base::Thread).

     ASSERT_TRUE(temp_dir_.CreateUniqueTempDir());
     CreateHistory();
   }