Mass migrate base::TaskPriority::BACKGROUND to BEST_EFFORT

This is:
  s/base::TaskPriority::BACKGROUND/base::TaskPriority::BEST_EFFORT/g
  git cl format
applied to //src.

This CL only touches cases explicitly prefixed with base:: to avoid
namespace conflicts should there be any. Will handle non-prefixed in a
follow-up CL.

[email protected]

Bug: 863341
Change-Id: Idad34a7ecca01644af155ab779eedb3a87544fc0
Reviewed-on: https://chromium-review.googlesource.com/1148546
Reviewed-by: Robert Liao <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#578415}
diff --git a/content/browser/file_url_loader_factory.cc b/content/browser/file_url_loader_factory.cc
index 44eb5224..822e0a0 100644
--- a/content/browser/file_url_loader_factory.cc
+++ b/content/browser/file_url_loader_factory.cc
@@ -672,7 +672,7 @@
     std::unique_ptr<FileURLLoaderObserver> observer,
     scoped_refptr<net::HttpResponseHeaders> extra_response_headers) {
   auto task_runner = base::CreateSequencedTaskRunnerWithTraits(
-      {base::MayBlock(), base::TaskPriority::BACKGROUND,
+      {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
        base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN});
   task_runner->PostTask(
       FROM_HERE,