[Background Fetch] Cleanup/fix thread safety

content/browser/blob_storage/chrome_blob_storage_context.cc

Index: content/browser/blob_storage/chrome_blob_storage_context.cc
diff --git a/content/browser/blob_storage/chrome_blob_storage_context.cc b/content/browser/blob_storage/chrome_blob_storage_context.cc
index 843dabcd08b2e271c102ed2545e7be25a046c85c..d99c8530c6293ea63908da85629ff7fb59e80c3d 100644
--- a/content/browser/blob_storage/chrome_blob_storage_context.cc
+++ b/content/browser/blob_storage/chrome_blob_storage_context.cc
@@ -81,6 +81,8 @@ ChromeBlobStorageContext::ChromeBlobStorageContext() {}
 
 ChromeBlobStorageContext* ChromeBlobStorageContext::GetFor(
     BrowserContext* context) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
   if (!context->GetUserData(kBlobStorageContextKeyName)) {
     scoped_refptr<ChromeBlobStorageContext> blob =
         new ChromeBlobStorageContext();