Use explicit operator bool for boolean testing of Oilpan pointer types.
Use explicit operator bool for boolean testing of Oilpan pointer types.

BUG=none

Review-Url: https://codereview.chromium.org/1973883006
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393704}

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