Change _ITERATOR_DEBUG_LEVEL to 1 on Release builds

VS2010 changed the default to 0 meaning weaker checks in our
release builds.  This commit changes it back again.

BUG=289691
NOTRY=true

Review URL: https://codereview.chromium.org/23464081

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226446 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226446