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..0cfefc31e75ad63756d04e25ba1a1fde53631571 100644
--- a/net/ssl/ssl_client_session_cache.h
+++ b/net/ssl/ssl_client_session_cache.h
@@ -46,8 +46,14 @@ class NET_EXPORT SSLClientSessionCache : public base::MemoryCoordinatorClient {
   size_t size() const;
 
   // Returns the session associated with |cache_key| and moves it to the front
-  // of the MRU list. Returns nullptr if there is none.
-  bssl::UniquePtr<SSL_SESSION> Lookup(const std::string& cache_key);
+  // of the MRU list. Returns nullptr if there is none. If |count| is non-null,
+  // |*count| will contain the number of times this session has been looked up
+  // (including this call).
+  bssl::UniquePtr<SSL_SESSION> Lookup(const std::string& cache_key, int* count);
+
+  // Resets the count returned by Lookup to 0 for the session associated with
+  // |cache_key|.
+  void ResetLookupCount(const std::string& cache_key);
 
   // 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
@@ -64,6 +70,15 @@ class NET_EXPORT SSLClientSessionCache : public base::MemoryCoordinatorClient {
   void DumpMemoryStats(base::trace_event::ProcessMemoryDump* pmd);
 
  private:
+  struct Entry {
+    Entry();
+    Entry(Entry&&);
+    ~Entry();
+
+    int lookups;
+    bssl::UniquePtr<SSL_SESSION> session;
+  };
+
   // base::MemoryCoordinatorClient implementation:
   void OnMemoryStateChange(base::MemoryState state) override;
 
@@ -79,7 +94,7 @@ 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