Oilpan: add release assert to catch nested GCs.
Oilpan: add release assert to catch nested GCs.

Sanity check that collectGarbage() isn't called while already GCing.

R=haraken
BUG=480852

Review URL: https://codereview.chromium.org/1141543002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@195310 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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