Oilpan: Add more checkHeader()s to Heap.cpp
Oilpan: Add more checkHeader()s to Heap.cpp

Headers sometimes get broken with lazy sweeping (because my implementation is still buggy).
We should have more checkHeader()s to verify we don't break them.

BUG=420515

Review URL: https://codereview.chromium.org/768493004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186154 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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