Add --js-flags=--expose-gc flag in OilpanGCTimes test.
Add --js-flags=--expose-gc flag in OilpanGCTimes test.

It enables renderers without DumpRenderTree to trigger Blink GCs.

BUG=438074, 420515

Review URL: https://codereview.chromium.org/964283002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#318888}

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