Remove non-const references from MemEntryImpl.

net/disk_cache/mem_entry_impl.h

Index: net/disk_cache/mem_entry_impl.h
diff --git a/net/disk_cache/mem_entry_impl.h b/net/disk_cache/mem_entry_impl.h
index ef91f6d7b0c211b4ac8f1b1d0c57878b77df77ff..b84cc39ef22d3e9093cb270fdf34c14f1a208859 100644
--- a/net/disk_cache/mem_entry_impl.h
+++ b/net/disk_cache/mem_entry_impl.h
@@ -82,11 +82,7 @@ class MemEntryImpl : public Entry {
     return parent_ ? kChildEntry : kParentEntry;
   }
 
-  std::string& key() {
-    return key_;
-  }
-
-  net::BoundNetLog& net_log() {
+  const net::BoundNetLog& net_log() {
     return net_log_;
   }