Fix DebugServer sequence check (#4326)

b/369965331

Fixes error "FATAL:sequenced_task_runner.cc(113)] Check failed:
current_default_handle. Error: This caller requires a sequenced context
(i.e. the current task needs to run from a SequencedTaskRunner)"

During the update of base, net and dependencies, there was a mass
replacement of uses of SingleThreadedTaskRunner with
SequencedTaskRunner. As part of that cleanup, we removed uses of
base::MessageLoop::current() with calls to
 base::SequencedTaskRunner::GetDefault().

In this particular case for DebugServer, it uses a base::Thread directly
rather than a SingleThreadedTaskRunner, so we can't compare our
thread.task_runner() with
base::SequencedTaskRunner::GetCurrentDefault(), but instead should use
task_runner()->RunsTasksInCurrentSequence() (as mentioned in Jelle's
comments in #2521)

cobalt/debug/remote/debug_web_server.cc

@@ -268,8 +268,7 @@ void DebugWebServer::OnDebugClientEvent(const std::string& method,
 
   // Debugger events occur on the thread of the web module the debugger is
   // attached to, so we must post to the server thread here.
-  if (base::SequencedTaskRunner::GetCurrentDefault() !=
-      http_server_thread_.task_runner()) {
+  if (!http_server_thread_.task_runner()->RunsTasksInCurrentSequence()) {
     http_server_thread_.task_runner()->PostTask(
         FROM_HERE, base::Bind(&DebugWebServer::OnDebugClientEvent,
                               base::Unretained(this), method, json_params));
@@ -286,8 +285,7 @@ void DebugWebServer::OnDebugClientEvent(const std::string& method,
 void DebugWebServer::OnDebugClientDetach(const std::string& reason) {
   // Debugger events occur on the thread of the web module the debugger is
   // attached to, so we must post to the server thread here.
-  if (base::SequencedTaskRunner::GetCurrentDefault() !=
-      http_server_thread_.task_runner()) {
+  if (!http_server_thread_.task_runner()->RunsTasksInCurrentSequence()) {
     http_server_thread_.task_runner()->PostTask(
         FROM_HERE, base::Bind(&DebugWebServer::OnDebugClientDetach,
                               base::Unretained(this), reason));