fix($sanitize): consider size attribute as valid/allowed attribute

The "size" attribute gets set on <font> elements when using HTML5 rich
text editors, or elements with the contenteditable attribute, that rely
on the 'fontSize' command (execCommand).

Closes angular#5522

src/ngSanitize/sanitize.js

@@ -206,7 +206,7 @@ var validAttrs = angular.extend({}, uriAttrs, makeMap(
     'abbr,align,alt,axis,bgcolor,border,cellpadding,cellspacing,class,clear,'+
     'color,cols,colspan,compact,coords,dir,face,headers,height,hreflang,hspace,'+
     'ismap,lang,language,nohref,nowrap,rel,rev,rows,rowspan,rules,'+
-    'scope,scrolling,shape,span,start,summary,target,title,type,'+
+    'scope,scrolling,shape,size,span,start,summary,target,title,type,'+
     'valign,value,vspace,width'));
 
 function makeMap(str) {