Add missing getter steps for HTMLScriptElement.src (#601)

Author: lukewarlow

spec/index.bs

@@ -1338,6 +1338,20 @@ Update the {{HTMLScriptElement/text}} setter steps algorithm as follows.
 
 #### The {{HTMLScriptElement/src}} IDL attribute #### {#the-src-idl-attribute}
 
+The {{HTMLScriptElement/src}} getter steps are:
+
+1. <ins>Let |element| be the result of running [=this=]'s [=get the element=].</ins>
+
+1. <ins>Let |contentAttributeValue| be the result of running [=this=]'s [=get the content attribute=].</ins>
+
+1. <ins>If |contentAttributeValue| is null, then return the empty string.</ins>
+
+1. <ins>Let |urlString| be the result of encoding-parsing-and-serializing a URL given |contentAttributeValue|, relative to |element|'s [=node document=].</ins>
+
+1. <ins>If |urlString| is not failure, then return |urlString|.</ins>
+
+1. <ins>Return |contentAttributeValue|, [=converted to a scalar value string=].
+
 The {{HTMLScriptElement/src}} setter steps are:
 
 1.  <ins>Let |value| be the result of calling [$Get Trusted Type compliant string$] with