SyncEngineBackend::OnMemoryDump should not assume Thread affinity.

This is ground work for experimenting with running ProfileSyncEngine's backend work on the ThreadPool.

This is done as the last mile of the chrome-wide effort to coalesce as much work as possible
to the ThreadPool.

For the change that will enable the experiment please refer to: https://chromium-review.googlesource.com/c/chromium/src/+/1856922

Bug: 1014464
Change-Id: I60e8081eefe40c90fc258aa9ac0467f910ca3356
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872410Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708173}

components/sync/driver/glue/sync_engine_backend.cc

@@ -10,8 +10,8 @@
 #include "base/files/file_util.h"
 #include "base/location.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/sequenced_task_runner.h"
 #include "base/threading/sequenced_task_runner_handle.h"
-#include "base/threading/thread_task_runner_handle.h"
 #include "base/trace_event/memory_dump_manager.h"
 #include "components/invalidation/public/invalidation_util.h"
 #include "components/invalidation/public/object_id_invalidation_map.h"
@@ -80,7 +80,7 @@ SyncEngineBackend::SyncEngineBackend(const std::string& name,
                                      const base::WeakPtr<SyncEngineImpl>& host)
     : name_(name), sync_data_folder_(sync_data_folder), host_(host) {
   DCHECK(host);
-  // This is constructed on the UI thread but used from the sync thread.
+  // This is constructed on the UI thread but used from another thread.
   DETACH_FROM_SEQUENCE(sequence_checker_);
 }
 
@@ -380,8 +380,9 @@ void SyncEngineBackend::DoInitialize(SyncEngine::InitParams params) {
   args.birthday = params.birthday;
   args.bag_of_chips = params.bag_of_chips;
   sync_manager_->Init(&args);
-  base::trace_event::MemoryDumpManager::GetInstance()->RegisterDumpProvider(
-      this, "SyncDirectory", base::ThreadTaskRunnerHandle::Get());
+  base::trace_event::MemoryDumpManager::GetInstance()
+      ->RegisterDumpProviderWithSequencedTaskRunner(
+          this, "SyncDirectory", base::SequencedTaskRunnerHandle::Get(), {});
 }
 
 void SyncEngineBackend::DoUpdateCredentials(
@@ -389,7 +390,7 @@ void SyncEngineBackend::DoUpdateCredentials(
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   // UpdateCredentials can be called during backend initialization, possibly
   // when backend initialization has failed but hasn't notified the UI thread
-  // yet. In that case, the sync manager may have been destroyed on the sync
+  // yet. In that case, the sync manager may have been destroyed on another
   // thread before this task was executed, so we do nothing.
   if (sync_manager_) {
     sync_manager_->UpdateCredentials(credentials);