Assert webkit platform in MemoryCache::prune().

This helps debug bug 272334.

BUG=272334

Review URL: https://chromiumcodereview.appspot.com/23102002

git-svn-id: svn://svn.chromium.org/blink/trunk@156046 bbb929c8-8fbe-4397-9dbb-9b2b20218538