Add a ScopedNSAutoreleasePool to TabModelTest.PersistSelectionChange

ios/chrome/browser/tabs/tab_model_unittest.mm

Index: ios/chrome/browser/tabs/tab_model_unittest.mm
diff --git a/ios/chrome/browser/tabs/tab_model_unittest.mm b/ios/chrome/browser/tabs/tab_model_unittest.mm
index 42b4af4061c625814bca4b1e0998a161dde4f6d4..3d6efae16d1fd67599c1920ec87848d2e8379ca2 100644
--- a/ios/chrome/browser/tabs/tab_model_unittest.mm
+++ b/ios/chrome/browser/tabs/tab_model_unittest.mm
@@ -831,6 +831,7 @@ TEST_F(TabModelTest, SetParentModel) {
 TEST_F(TabModelTest, PersistSelectionChange) {
   TestChromeBrowserState::Builder test_cbs_builder;
   auto chrome_browser_state = test_cbs_builder.Build();
+  base::mac::ScopedNSAutoreleasePool pool;

[rohitrao (ping after 24h), 2017/02/06 17:31:46]

Can you please add a comment explaining why this pool is necessary?

[sdefresne, 2017/02/06 17:46:43]

Done.

 
   NSString* stashPath =
       base::SysUTF8ToNSString(chrome_browser_state->GetStatePath().value());
@@ -868,6 +869,8 @@ TEST_F(TabModelTest, PersistSelectionChange) {
                browserState:chrome_browser_state.get()]);
   EXPECT_EQ(model.get().currentTab, [model tabAtIndex:1]);
   [model browserStateDestroyed];
+  model.reset();
+  base::RunLoop().RunUntilIdle();

[rohitrao (ping after 24h), 2017/02/06 17:31:46]

Why is this necessary?  Does this drain the pool?  Could you just delete the
pool here instead?

[sdefresne, 2017/02/06 17:46:43]

This is unrelated to draining the pool.

It is there because restoring a session sends a task to the UI thread with a
callback on the UI thread (see https://codereview.chromium.org/2674293002/ where
I make this callback thread-safe). So this fixes another possible source of
flaky-ness.

I've put a comment why this is there.

 
   // Clean up.
   EXPECT_TRUE([[NSFileManager defaultManager] removeItemAtPath:stashPath