Remove non-const references from MemEntryImpl.

Review URL: https://chromiumcodereview.appspot.com/23530064

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@223510 0039d316-1c4b-4281-b951-d872f2087c98

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_;
   }