[web-animations-1] Add Stephen McGruger to list of editors

As per resolution: https://lists.w3.org/Archives/Public/www-style/2018Jul/0020.html

Welcome Stephen!

Author: birtles

web-animations-1/Overview.bs

@@ -87,6 +87,7 @@ Repository: w3c/csswg-drafts
 
 Editor: Brian Birtles 43194, Mozilla, bbirtles@mozilla.com
 Editor: Robert Flack 98451, Google Inc, flackr@google.com
+Editor: Stephen McGruer 96463, Google Inc, smcgruer@google.com
 Editor: Antoine Quint 51377, Apple Inc, graouts@apple.com
 Former Editor: Shane Stephens 47691, Google Inc, shans@google.com
 Former Editor: Douglas Stockwell, Google Inc, dstockwell@google.com