Skip updating rank in OnExternalCacheHit if cache is uninitialized or disabled.

BUG=92913
TEST=net_unittests


Review URL: http://codereview.chromium.org/7650022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@96882 0039d316-1c4b-4281-b951-d872f2087c98

net/disk_cache/backend_impl.cc

@@ -683,6 +683,9 @@ void BackendImpl::SyncEndEnumeration(void* iter) {
 }
 
 void BackendImpl::SyncOnExternalCacheHit(const std::string& key) {
+  if (disabled_)
+    return;
+
   uint32 hash = Hash(key);
   bool error;
   EntryImpl* cache_entry = MatchEntry(key, hash, false, Addr(), &error);