Make MemoryCache's LRUList manipulation private

Changes to the LRU lists are hidden behind a call to MemoryCache::update(), which adjusts sizes, access count, and LRU list position as appropriate. All of these are to some degree interrelated, since we keep multiple LRU lists and select one to use based access count and resource size.

Also, remove the Resource::inCache() bit in favor of a
MemoryCache::contains()

BUG=none

Review URL: https://codereview.chromium.org/164333008

git-svn-id: svn://svn.chromium.org/blink/trunk@169748 bbb929c8-8fbe-4397-9dbb-9b2b20218538