alternate ntp: fix crash in RecentTabsSubMenuModel::GetURLAndTitleForItemAtIndex()

cause: GetURLAndTitleForItemAtIndex() is only called on mac for all menu items in RecentTabsSubMenuModel.  the code didn't check for disabled items, which do not have corresponding items in the internal storage vectors.  this causes the access into out-of-bounds storage vector.

fix: make sure all checks of command_id's have lower and upper bounds, as all disabled items have higher command_id's than enabled items.

- added test.

BUG=306821,256750
TEST=crash in bug 306821 doesn't happen.
R=sky@chromium.org

Review URL: https://codereview.chromium.org/27116002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228514 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228514