Adds LogDnsClient::NotifyWhenNotThrottled

components/certificate_transparency/log_dns_client.h

Index: components/certificate_transparency/log_dns_client.h
diff --git a/components/certificate_transparency/log_dns_client.h b/components/certificate_transparency/log_dns_client.h
index 2786307d64441de29c431eb3f65097ce49f06757..ded22255d2dce5fb5454efa03545ed6946cb4935 100644
--- a/components/certificate_transparency/log_dns_client.h
+++ b/components/certificate_transparency/log_dns_client.h
@@ -53,10 +53,17 @@ class LogDnsClient : public net::NetworkChangeNotifier::DNSObserver {
 
   // Called by NetworkChangeNotifier when the DNS config is first read.
   // The DnsClient's config will be updated in response.
   void OnInitialDNSConfigRead() override;
 
+  // Registers a callback to be invoked when the number of concurrent queries
+  // falls below the limit defined by |max_concurrent_queries| (passed to the
+  // constructor of LogDnsClient). This callback will fire once and then be
+  // unregistered. Should only be used if QueryAuditProof() returns
+  // net::ERR_TEMPORARILY_THROTTLED.
+  void NotifyWhenNotThrottled(const base::Closure& callback);
+
   // Queries a CT log to retrieve an audit proof for the leaf with |leaf_hash|.
   // The log is identified by |domain_for_log|, which is the DNS name used as a
   // suffix for all queries.
   // The |leaf_hash| is the SHA-256 Merkle leaf hash (see RFC6962, section 2.1).
   // The size of the CT log tree, for which the proof is requested, must be
@@ -110,10 +117,12 @@ class LogDnsClient : public net::NetworkChangeNotifier::DNSObserver {
   // the end and lookups will typically yield entries at the beginning,
   // std::list is an efficient choice.
   std::list<std::unique_ptr<AuditProofQuery>> audit_proof_queries_;
   // The maximum number of queries that can be in flight at one time.
   size_t max_concurrent_queries_;
+  // Callbacks to invoke when the number of concurrent queries is at its limit.
+  std::list<base::Closure> not_throttled_callbacks_;
   // Creates weak_ptrs to this, for callback purposes.
   base::WeakPtrFactory<LogDnsClient> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(LogDnsClient);
 };