Allow null other.m_parent in TraceWrapperMember::operator= if other is nullptr.

This CL fixes the DCHECK condition in TraceWrapperMember::operator= to allow
assignment of null TraceWrapperMember.

This CL allows TraceWrapperMember to be used in HeapHash{Set,Map}, as
HashTable::rehash operations involve assignment of empty==null TraceWrapperMembers.

BUG=594639, 707123

Review-Url: https://codereview.chromium.org/2802593002
Cr-Commit-Position: refs/heads/master@{#462044}
2 files changed