Adds some debugging to figure out if TabStrip is being passed invalid index

Syzy says we're accessing beyond the size of TabStrip::tabs_. This
adds some code in hopes of isolating that.

BUG=294353
TEST=none
R=ben@chromium.org

Review URL: https://codereview.chromium.org/25021007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226247 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226247