Oilpan: BaseObjectHeader::m_size doesn't need to be volatile
Oilpan: BaseObjectHeader::m_size doesn't need to be volatile

Now that we don't support parallel marking, BaseObjectHeader::m_size doesn't need to be volatile.

BUG=420515

Review URL: https://codereview.chromium.org/810973002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@187352 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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