Use TaskPriority::USER_VISIBLE for BlobMemoryController's file_runner

Bug: 989868,924416
Change-Id: I7b19d9e9332df1472e0511a9a410d4e3331e9f6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730832Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683361}

content/browser/blob_storage/chrome_blob_storage_context.cc

@@ -119,7 +119,7 @@ ChromeBlobStorageContext* ChromeBlobStorageContext::GetFor(
     // disk on the storage context.
     if (!context->IsOffTheRecord() && io_thread_valid) {
       file_task_runner = base::CreateTaskRunnerWithTraits(
-          {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
+          {base::MayBlock(), base::TaskPriority::USER_VISIBLE,
            base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN});
       // Removes our old blob directories if they exist.
       BrowserThread::PostBestEffortTask(