Add DCHECKs to BlobTaskProxy to validate the intended sequencing restrictions.

The calls to BelongsToCurrentThread appear to be no-ops at the moment, and
assume that they were intended to be DCHECKS, so a quick CL to fix.

Change-Id: I33a6c8e05312b528d5a4e89fd18a0cff6281cd28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762238
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720592}

components/download/internal/background_service/blob_task_proxy.cc

@@ -36,11 +36,12 @@ BlobTaskProxy::BlobTaskProxy(
 }
 
 BlobTaskProxy::~BlobTaskProxy() {
-  io_task_runner_->BelongsToCurrentThread();
+  CHECK(io_task_runner_->BelongsToCurrentThread());
 }
 
 void BlobTaskProxy::InitializeOnIO(BlobContextGetter blob_context_getter) {
-  io_task_runner_->BelongsToCurrentThread();
+  DCHECK(io_task_runner_->BelongsToCurrentThread());
+
   blob_storage_context_ = blob_context_getter.Run();
 }
 
@@ -56,7 +57,7 @@ void BlobTaskProxy::SaveAsBlob(std::unique_ptr<std::string> data,
 
 void BlobTaskProxy::SaveAsBlobOnIO(std::unique_ptr<std::string> data,
                                    BlobDataHandleCallback callback) {
-  io_task_runner_->BelongsToCurrentThread();
+  DCHECK(io_task_runner_->BelongsToCurrentThread());
 
   // Build blob data. This has to do a copy into blob's internal storage.
   std::string blob_uuid = base::GenerateGUID();
@@ -73,7 +74,7 @@ void BlobTaskProxy::SaveAsBlobOnIO(std::unique_ptr<std::string> data,
 
 void BlobTaskProxy::BlobSavedOnIO(BlobDataHandleCallback callback,
                                   storage::BlobStatus status) {
-  io_task_runner_->BelongsToCurrentThread();
+  DCHECK(io_task_runner_->BelongsToCurrentThread());
 
   // Relay BlobDataHandle and |status| back to main thread.
   auto cb =