Fix use after free in memory only backend.

net/disk_cache/memory/mem_entry_impl.cc

Index: net/disk_cache/memory/mem_entry_impl.cc
diff --git a/net/disk_cache/memory/mem_entry_impl.cc b/net/disk_cache/memory/mem_entry_impl.cc
index c28a7fab748d565877004f12331eb6db71efbfb4..bcbebb3cf2f8b7992a307489f828fc5d3025c763 100644
--- a/net/disk_cache/memory/mem_entry_impl.cc
+++ b/net/disk_cache/memory/mem_entry_impl.cc
@@ -103,16 +103,10 @@ void MemEntryImpl::Open() {
 }
 
 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 {