The `Broadcast`/`ResendWalletTransactions` interface is strange:

- it's the node saying to the wallet "Try to submit your unconfirmed transactions to the mempool again"
- it's called in one place by the node, in `SendMessages()` here: https://github.com/bitcoin/bitcoin/blob/c033c4b5cef89a654e4d9d5c5f9bd823871b068b/src/net_processing.cpp#L3515. `SendMessages()` is called on the message handling thread on a loop.
- the actual scheduling of when the wallet sends messages is handled in the `ResendWalletTransactions()` function itself, using the `nNextResend` and `nLastResend` wallet globals (which could as easily be static variables in `ResendWalletTransactions()`).

Therefore the only use of `Broadcast()` is to periodically prod the wallet. The wallet itself decides whether it's time to resend transactions. This could be done by scheduling regular calls to `ResendWalletTransactions()`, in the same way that we schedule regular wallet flushes here: https://github.com/bitcoin/bitcoin/blob/c033c4b5cef89a654e4d9d5c5f9bd823871b068b/src/wallet/init.cpp#L215.

A couple of minor implementation details:

- the call to `Broadcast()` in `SendMessages()` is protected by an if statement `if (!fReindex && !fImporting && !IsInitialBlockDownload()) {`. Those tests could be moved into `ResendWalletTransactions()` (through the `Chain` interface).
- The `Broadcast` notification passes in the `nTimeBestReceived` time of the best block. That could be fetched by `ResendWalletTransactions()` through the `Chain` interface.
{
    std::map<CTxDestination, CAmount> balances;
    {
        LOCK(cs_wallet);
        for (const auto& walletEntry : mapWallet)
        {
            const CWalletTx& wtx = walletEntry.second;
            if (!wtx.IsTrusted(locked_chain))
                continue;
            if (wtx.IsImmatureCoinBase(locked_chain))
                continue;
            int nDepth = wtx.GetDepthInMainChain(locked_chain);
            if (nDepth < (wtx.IsFromMe(ISMINE_ALL) ? 0 : 1))
                continue;
            for (unsigned int i = 0; i < wtx.tx->vout.size(); i++)
            {
                CTxDestination addr;
                if (!IsMine(wtx.tx->vout[i]))
                    continue;
                if(!ExtractDestination(wtx.tx->vout[i].scriptPubKey, addr))
                    continue;
                CAmount n = IsSpent(locked_chain, walletEntry.first, i) ? 0 : wtx.tx->vout[i].nValue;
                if (!balances.count(addr))
                    balances[addr] = 0;