Oilpan: Prepare to move MemoryCache and MemoryCahce::MemoryCacheEntry to Oilpan heap.

Also, this CL makes MemoryCacheLRUList traceable.

setMemoryCacheForTesting was renamed to replaceMemoryCacheFor Testing,
and it returns PassOwnPtr<> for the previous one.

This is a prepration to move Resource hierarchy to Oilpan heap.

BUG=393516

Review URL: https://codereview.chromium.org/416803002

git-svn-id: svn://svn.chromium.org/blink/trunk@178915 bbb929c8-8fbe-4397-9dbb-9b2b20218538