Disk cache: fix broken UMA reports.

net/disk_cache/entry_impl.cc

Index: net/disk_cache/entry_impl.cc
===================================================================
--- net/disk_cache/entry_impl.cc	(revision 13405)
+++ net/disk_cache/entry_impl.cc	(working copy)
@@ -539,6 +539,9 @@
 
 bool EntryImpl::IsDirty(int32 current_id) {
   DCHECK(node_.HasData());
+  // We are checking if the entry is valid or not. If there is a pointer here,
+  // |dirty| has to be the id of the cache that is using the entry (the one
+  // that created the pointer), 0 is not a valid id.
   if (node_.Data()->pointer && !node_.Data()->dirty)
     return true;