Fix use after free in memory only backend.

net/disk_cache/memory/mem_entry_impl.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/disk_cache/memory/mem_entry_impl.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 85 matching lines @@
 
 void MemEntryImpl::Open() {
   // Only a parent entry can be opened.
   DCHECK_EQ(PARENT_ENTRY, type());
   ++ref_count_;
   DCHECK_GE(ref_count_, 1);
   DCHECK(!doomed_);
 }
 
 bool MemEntryImpl::InUse() const {
-  if (type() == PARENT_ENTRY) {
-    return ref_count_ > 0;
-  } else {
-    // TODO(gavinp): Can't this just be a DCHECK? How would ref_count_ not be
-    // zero?
+  if (type() == CHILD_ENTRY)
+    return parent_->InUse();
 
-    // A child entry is never in use. Thus one can always be evicted, even while
-    // its parent entry is open and in use.
-    return false;
-  }
+  return ref_count_ > 0;
 }
 
 int MemEntryImpl::GetStorageSize() const {
   int storage_size = static_cast<int32_t>(key_.size());
   for (const auto& i : data_)
     storage_size += i.size();
   return storage_size;
 }
 
 void MemEntryImpl::UpdateStateOnUse(EntryModified modified_enum) {
@@ skipping 462 matching lines @@
          scanned_len += first_pos - current_child_offset;
          break;
       }
     }
     scanned_len += kMaxSparseEntrySize - current_child_offset;
   }
   return scanned_len;
 }
 
 }  // namespace disk_cache