Replace deprecated base::NonThreadSafe in content/browser/appcache in favor of SequenceChecker.

content/browser/appcache/appcache_request.cc

Index: content/browser/appcache/appcache_request.cc
diff --git a/content/browser/appcache/appcache_request.cc b/content/browser/appcache/appcache_request.cc
index 62c6d74e8cf63cdaa743a364f96ed2cd2b984a23..6fc31ecd14441e3b19878da983dab8fc4abf3c80 100644
--- a/content/browser/appcache/appcache_request.cc
+++ b/content/browser/appcache/appcache_request.cc
@@ -8,6 +8,10 @@
 
 namespace content {
 
+AppCacheRequest::~AppCacheRequest() {
+  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
+}
+
 // static
 bool AppCacheRequest::IsSchemeAndMethodSupportedForAppCache(
     const AppCacheRequest* request) {