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 544559480588c9f4d42cd6da1f0f2eccc5cbc2eb..4f07a3e18f9fba0b120733b562e7c0b349c44f8a 100644
--- a/net/disk_cache/simple/simple_entry_impl.h
+++ b/net/disk_cache/simple/simple_entry_impl.h
@@ -247,7 +247,7 @@ 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 net::CacheType cache_type_;
   const scoped_refptr<base::TaskRunner> worker_pool_;
   const base::FilePath path_;
@@ -267,6 +267,8 @@ 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