Move out MemoryCacheEntry and LRUList from MemoryCache class.

They were inner class/struct of MemoryCache. We'd like to move them out because
LRUList should have VectorTraits specialiazation, and VectorTraits
specialiazation don't support inner classes/structs.

Also, this CL renames LRUList to MemoryCacheLRUList because LRUList is too
generic.

This CL is a preparatio to make LRUList traceable.

BUG=393516

Review URL: https://codereview.chromium.org/416263003

git-svn-id: svn://svn.chromium.org/blink/trunk@178900 bbb929c8-8fbe-4397-9dbb-9b2b20218538