Revert "Move MemoryCache implementation details out of Resource"

This reverts commit 3af7e850c92091b73b8885019259951563741791.

It is the likely cause of crbug.com/344620 (and the easier to revert of the
two candidates).

BUG=344620
TBR=abarth

Review URL: https://codereview.chromium.org/171443002

git-svn-id: svn://svn.chromium.org/blink/trunk@167367 bbb929c8-8fbe-4397-9dbb-9b2b20218538