SSLClientSessionCache: Log number of times Lookup is called per Session.

net/ssl/ssl_client_session_cache.h

Index: net/ssl/ssl_client_session_cache.h
diff --git a/net/ssl/ssl_client_session_cache.h b/net/ssl/ssl_client_session_cache.h
index 4a82010badda3722a6d318ff74951016a155bfc2..bb658c229c21fa176fc7be9795c54207c8483c8d 100644
--- a/net/ssl/ssl_client_session_cache.h
+++ b/net/ssl/ssl_client_session_cache.h
@@ -49,6 +49,12 @@ class NET_EXPORT SSLClientSessionCache : public base::MemoryCoordinatorClient {
   // of the MRU list. Returns nullptr if there is none.
   bssl::UniquePtr<SSL_SESSION> Lookup(const std::string& cache_key);
 
+  // If Lookup returns a non-null pointer, this method must be called once the
+  // TLS handshake done with the session from Lookup has completed. If
+  // |should_log| is true, the number of concurrent lookups will be logged to
+  // UMA once the count has reached 0.
+  void DecrementLookupCount(const std::string& cache_key, bool should_log);
+
   // Inserts |session| into the cache at |cache_key|. If there is an existing
   // one, it is released. Every |expiration_check_count| calls, the cache is
   // checked for stale entries.
@@ -64,6 +70,18 @@ class NET_EXPORT SSLClientSessionCache : public base::MemoryCoordinatorClient {
   void DumpMemoryStats(base::trace_event::ProcessMemoryDump* pmd);
 
  private:
+  struct Entry {
+   public:
+    Entry();
+    Entry(Entry&&);
+    ~Entry();
+
+    size_t active_lookups;
+    size_t max_lookups;
+    bool should_log;
+    bssl::UniquePtr<SSL_SESSION> session;
+  };
+
   // base::MemoryCoordinatorClient implementation:
   void OnMemoryStateChange(base::MemoryState state) override;
 
@@ -79,7 +97,7 @@ class NET_EXPORT SSLClientSessionCache : public base::MemoryCoordinatorClient {
 
   std::unique_ptr<base::Clock> clock_;
   Config config_;
-  base::HashingMRUCache<std::string, bssl::UniquePtr<SSL_SESSION>> cache_;
+  base::HashingMRUCache<std::string, Entry> cache_;
   size_t lookups_since_flush_;
 
   // TODO(davidben): After https://crbug.com/458365 is fixed, replace this with