Oilpan: Avoid crashes in MemoryCache-related tests.
Oilpan: Avoid crashes in MemoryCache-related tests.

r179210 added GC to runHelper(). However it exposed MemoryCache-related bugs in
Oilpan. We'll investigate the root cause soon.

BUG=393516
TBR=yhirano@chromium.org,oilpan-reviews@chromium.org

Review URL: https://codereview.chromium.org/437503002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@179296 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs