Fix possible naming conflict in macro.

The ObserverList macro (FOR_EACH_OBSERVER) uses an iterator named |it|, which can in some cases conflict with locally defined variables.

Make the iterator variable name slightly more unique

BUG=None
R=jar@chromium.org

Review URL: https://codereview.chromium.org/23672052

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224985 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224985