Crash in brave_rewards::RunDBTransactionOnFileTaskRunner

[iefremov]

it seems the posted task still can observe bad ledger_database_ in RunDBTransactionOnFileTaskRunner

happens in 90.1.24.76 that already has recent threading fixes

cc @emerick @zenparsing @bridiver

https://brave.sp.backtrace.io/p/brave/debug?filters=(_deleted%3D0%2C(callstack.functions%2Ccontains%2CLedgerDatabase))&debug=(%22104302c%22,0,0)

[ 00 ] 0x7f8179308344
[ 01 ] readDbPage
[ 02 ] getPageNormal
[ 03 ] getPageMMap
[ 04 ] getAndInitPage
[ 05 ] moveToRoot
[ 06 ] sqlite3BtreeMovetoUnpacked
[ 07 ] sqlite3VdbeExec
[ 08 ] chrome_sqlite3_step
[ 09 ] sql::Statement::StepInternal()
[ 10 ] ledger::LedgerDatabaseImpl::RunTransaction(mojo::StructPtr<ledger::mojom::DBTransaction>, ledger::mojom::DBCommandResponse*)
[ 11 ] brave_rewards::RunDBTransactionOnFileTaskRunner(mojo::StructPtr<ledger::mojom::DBTransaction>, ledger::LedgerDatabase*)
[ 12 ] base::internal::Invoker<base::internal::BindState<mojo::StructPtr<ads_database::mojom::DBCommandResponse> (*)(mojo::StructPtr<ads_database::mojom::DBTransaction>, ads::Database*), mojo::StructPtr<ads_database::mojom::DBTransaction>, ads::Database*>, mojo::StructPtr<ads_database::mojom::DBCommandResponse> ()>::RunOnce(base::internal::BindStateBase*)
[ 13 ] void base::internal::ReturnAsParamAdapter<mojo::StructPtr<ads_database::mojom::DBCommandResponse> >(base::OnceCallback<mojo::StructPtr<ads_database::mojom::DBCommandResponse> ()>, std::__1::unique_ptr<mojo::StructPtr<ads_database::mojom::DBCommandResponse>, std::__1::default_delete<mojo::StructPtr<ads_database::mojom::DBCommandResponse> > >*)
[ 14 ] base::internal::Invoker<base::internal::BindState<void (*)(base::OnceCallback<bool ()>, std::__1::unique_ptr<bool, std::__1::default_delete<bool> >*), base::OnceCallback<bool ()>, std::__1::unique_ptr<bool, std::__1::default_delete<bool> >*>, void ()>::RunOnce(base::internal::BindStateBase*)
[ 15 ] base::(anonymous namespace)::PostTaskAndReplyRelay::RunTaskAndPostReply(base::(anonymous namespace)::PostTaskAndReplyRelay)

[zenparsing]

The ledger_database_ pointer is generally deleted with DeleteSoon, except for one case here. I'm not sure if the crash is arising from this line or not, but one solution would be to use base::OnTaskRunnerDeleter to ensure that the pointer is always deleted on the "file task runner", instead of calling DeleteSoon explicitly. @iefremov @bridiver, would that be a good path forward, or is there another approach that we should take?