Oilpan: retire dated DataRef<> GC_PLUGIN_IGNORE().
Oilpan: retire dated DataRef<> GC_PLUGIN_IGNORE().

R=haraken
BUG=553613

Review URL: https://codereview.chromium.org/1577753002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368560}

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