Remove some appending behavior from GN.

Remove operator += and -= to append/remove a non-list to/from a list. Now you will have to wrap a single item in [] if you want to add or remove it. This makes the syntax more uniform since people were doing it randomly with and without [].

This removes the error of assigning to an unused variable. This ended up being too much of a bother, and causing errors in cases were you wouldn't expect it.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/136723008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247202 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247202