Track entries pending Doom in SimpleCache backend.

net/disk_cache/simple/simple_entry_impl.h

Index: net/disk_cache/simple/simple_entry_impl.h
diff --git a/net/disk_cache/simple/simple_entry_impl.h b/net/disk_cache/simple/simple_entry_impl.h
index 7daf381541a32fd07486f83c29bf60223c4c5520..31acad70a0c0fada7e2d7e421672622cb9003e26 100644
--- a/net/disk_cache/simple/simple_entry_impl.h
+++ b/net/disk_cache/simple/simple_entry_impl.h
@@ -245,7 +245,7 @@ class SimpleEntryImpl : public Entry, public base::RefCounted<SimpleEntryImpl>,
   // thread, in all cases. |io_thread_checker_| documents and enforces this.
   base::ThreadChecker io_thread_checker_;
 
-  base::WeakPtr<SimpleBackendImpl> backend_;
+  const base::WeakPtr<SimpleBackendImpl> backend_;
   const scoped_refptr<base::TaskRunner> worker_pool_;
   const base::FilePath path_;
   const uint64 entry_hash_;
@@ -264,6 +264,8 @@ class SimpleEntryImpl : public Entry, public base::RefCounted<SimpleEntryImpl>,
   // notify the backend when this entry not used by any callers.
   int open_count_;
 
+  bool doomed_;
+
   State state_;
 
   // When possible, we compute a crc32, for the data in each entry as we read or