Making WebCore::MemoryCache 64-bit safe

BUG=319703
R=eseidel@chromium.org, japhet@chromium.org

Review URL: https://codereview.chromium.org/104703005

git-svn-id: svn://svn.chromium.org/blink/trunk@163791 bbb929c8-8fbe-4397-9dbb-9b2b20218538