Fixup MessageListView API.

This change removes the possibility of a few bugs, which are likely to
have caused the crash from the referenced bug.  The two main bugs are:
removing brittle index-based changes to MessageListView, and properly
checking the return values for GetIndexOf when necessary.

BUG=334732

Review URL: https://codereview.chromium.org/156773002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250243 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250243