Fix ~GCMStoreImpl() performing IO on the UI thread

This is needed to enable DestroyProfileOnBrowserClose by default in
automated tests (crrev.com/c/2503629).

GCMStoreImpl::Backend owns an sql::Database object, which may perform
IO when destroyed. Release it on the blocking task runner, so it
doesn't cause issues when the DestroyProfileOnBrowserClose flag is on.

Bug: 88586
Change-Id: I657ae488c6a55c1608bf45466ea6a3bc0153786f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2511250
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826278}

components/gcm_driver/gcm_client_impl_unittest.cc

@@ -464,6 +464,9 @@ void GCMClientImplTest::SetUp() {
 }
 
 void GCMClientImplTest::TearDown() {
+  gcm_client_.reset();
+  PumpLoopUntilIdle();
+  testing::Test::TearDown();
 }
 
 void GCMClientImplTest::SetFeatureParams(const base::Feature& feature,

google_apis/gcm/engine/gcm_store_impl.cc

@@ -1203,7 +1203,11 @@ GCMStoreImpl::GCMStoreImpl(
                            std::move(encryptor))),
       blocking_task_runner_(blocking_task_runner) {}
 
-GCMStoreImpl::~GCMStoreImpl() {}
+GCMStoreImpl::~GCMStoreImpl() {
+  // |backend_| owns an sql::Database object, which may perform IO when
+  // |destroyed.
+  blocking_task_runner_->ReleaseSoon(FROM_HERE, std::move(backend_));
+}
 
 void GCMStoreImpl::Load(StoreOpenMode open_mode, LoadCallback callback) {
   blocking_task_runner_->PostTask(

google_apis/gcm/engine/gcm_store_impl_unittest.cc

@@ -51,6 +51,8 @@ class GCMStoreImplTest : public testing::Test {
   GCMStoreImplTest();
   ~GCMStoreImplTest() override;
 
+  void TearDown() override;
+
   std::unique_ptr<GCMStoreImpl> BuildGCMStore();
   void LoadGCMStore(GCMStoreImpl* gcm_store,
                     std::unique_ptr<GCMStore::LoadResult>* result_dst);
@@ -85,6 +87,10 @@ GCMStoreImplTest::GCMStoreImplTest()
 
 GCMStoreImplTest::~GCMStoreImplTest() {}
 
+void GCMStoreImplTest::TearDown() {
+  PumpLoop();
+}
+
 std::unique_ptr<GCMStoreImpl> GCMStoreImplTest::BuildGCMStore() {
   // Pass an non-existent directory as store path to match the exact behavior in
   // the production code. Currently GCMStoreImpl checks if the directory exists

google_apis/gcm/engine/mcs_client_unittest.cc

@@ -130,6 +130,7 @@ class MCSClientTest : public testing::Test {
   ~MCSClientTest() override;
 
   void SetUp() override;
+  void TearDown() override;
 
   void BuildMCSClient();
   void InitializeClient();
@@ -173,7 +174,7 @@ class MCSClientTest : public testing::Test {
   base::SimpleTestClock clock_;
 
   base::ScopedTempDir temp_directory_;
-  base::test::SingleThreadTaskEnvironment task_environment_;
+  base::test::TaskEnvironment task_environment_;
   std::unique_ptr<base::RunLoop> run_loop_;
   std::unique_ptr<GCMStore> gcm_store_;
 
@@ -208,6 +209,12 @@ void MCSClientTest::SetUp() {
   testing::Test::SetUp();
 }
 
+void MCSClientTest::TearDown() {
+  gcm_store_.reset();
+  task_environment_.RunUntilIdle();
+  testing::Test::TearDown();
+}
+
 void MCSClientTest::BuildMCSClient() {
   gcm_store_.reset(
       new GCMStoreImpl(temp_directory_.GetPath(),