[Background Fetch] Cleanup/fix thread safety

content/browser/blob_storage/chrome_blob_storage_context.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/blob_storage/chrome_blob_storage_context.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/feature_list.h"
@@ skipping 63 matching lines @@
  private:
   std::unique_ptr<storage::BlobDataHandle> handle_;
 };
 
 }  // namespace
 
 ChromeBlobStorageContext::ChromeBlobStorageContext() {}
 
 ChromeBlobStorageContext* ChromeBlobStorageContext::GetFor(
     BrowserContext* context) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
   if (!context->GetUserData(kBlobStorageContextKeyName)) {
     scoped_refptr<ChromeBlobStorageContext> blob =
         new ChromeBlobStorageContext();
     context->SetUserData(
         kBlobStorageContextKeyName,
         base::MakeUnique<UserDataAdapter<ChromeBlobStorageContext>>(
             blob.get()));
 
     // Check first to avoid memory leak in unittests.
     bool io_thread_valid = BrowserThread::IsMessageLoopValid(BrowserThread::IO);
@@ skipping 132 matching lines @@
     std::unique_ptr<storage::BlobDataHandle> handle =
         blob_context->GetBlobDataFromUUID(element.blob_uuid());
     if (!handle)
       return false;
     blob_handles->push_back(std::move(handle));
   }
   return true;
 }
 
 }  // namespace content