Don't discard stability stats in incognito

Aligns stability metric handling with histogram handling, and ensures
that incognito use doesn't skew understanding of stability.

BUG=336610

Review URL: https://codereview.chromium.org/154743006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256119 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256119