Cleanup: Move PreferredCacheSize() to cache_util.h

BUG=173425

Review URL: https://codereview.chromium.org/50373004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236022 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236022