Removed unneeded taskrunner in proto db provider

ProtoDatabaseProvider had an unused refptr to a TaskRunner, this CL
removes it and updates some comments.

Bug: 870813
Change-Id: I3980842c4741f2e84f93dcd8070c338b08cf3b28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538996
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644661}

components/leveldb_proto/public/proto_database_provider.cc

@@ -26,10 +26,7 @@ const char kSharedProtoDatabaseDirectory[] = "shared_proto_db";
 
 ProtoDatabaseProvider::ProtoDatabaseProvider(const base::FilePath& profile_dir)
     : profile_dir_(profile_dir),
-      task_runner_(base::CreateSequencedTaskRunnerWithTraits(
-          {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
-           base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN})),
-      creation_sequence_(base::SequencedTaskRunnerHandle::Get()),
+      client_task_runner_(base::SequencedTaskRunnerHandle::Get()),
       weak_factory_(this) {}
 
 ProtoDatabaseProvider::~ProtoDatabaseProvider() = default;

components/leveldb_proto/public/proto_database_provider.h

@@ -62,14 +62,11 @@ class ProtoDatabaseProvider : public KeyedService {
   base::FilePath profile_dir_;
   scoped_refptr<SharedProtoDatabase> db_;
   base::Lock get_db_lock_;
-  // The SequencedTaskRunner used to ensure thread-safe behaviour for
-  // GetSharedDBInstance when called from multiple clients.
-  scoped_refptr<base::SequencedTaskRunner> task_runner_;
 
-  // We store the creation sequence because we want to use that to make requests
-  // to the main provider that rely on WeakPtrs from this, so they're all
-  // invalidated/checked on the same sequence.
-  scoped_refptr<base::SequencedTaskRunner> creation_sequence_;
+  // We store the creation sequence because the SharedProtoDatabaseProvider uses
+  // it to make requests to the main provider that rely on WeakPtrs from here,
+  // so they're all invalidated/checked on the same sequence.
+  scoped_refptr<base::SequencedTaskRunner> client_task_runner_;
 
   base::WeakPtrFactory<ProtoDatabaseProvider> weak_factory_;
 
@@ -84,7 +81,7 @@ std::unique_ptr<ProtoDatabase<P, T>> ProtoDatabaseProvider::GetDB(
   return base::WrapUnique(new ProtoDatabaseImpl<P, T>(
       db_type, unique_db_dir, task_runner,
       base::WrapUnique(new SharedProtoDatabaseProvider(
-          creation_sequence_, weak_factory_.GetWeakPtr()))));
+          client_task_runner_, weak_factory_.GetWeakPtr()))));
 }
 
 }  // namespace leveldb_proto