Safeguard against runaway memory consumption by dead resources with deferred MemoryCache pruning

This is a follow-up to r156156 to reduce possibility of largely increased memory consumption as a result of the new lazier cache pruning approach.

TEST=MemoryCache webkit unit tests
BUG=271588

Review URL: https://chromiumcodereview.appspot.com/23140006

git-svn-id: svn://svn.chromium.org/blink/trunk@156577 bbb929c8-8fbe-4397-9dbb-9b2b20218538