Add logging to help understand list-index issues on page cycler

BUG=348337
TBR=tonyg
NOTRY=True

Review URL: https://codereview.chromium.org/185533002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254419 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254419