After the last index entry, we put an extra entry (id of 0, offset
pointing to where the next entry would start) so we can compute
the size of the last index entry.

This saves 4 bytes per item in data pack files, which is about 10k per
locale pak file.  When compressed, this saves about 237k.

BUG=76285

Review URL: http://codereview.chromium.org/7604012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@96415 0039d316-1c4b-4281-b951-d872f2087c98