[css-variables] Make delete() return a boolean, like Map does.

Author: tabatkins

css-variables/Overview.html

@@ -674,7 +674,7 @@ <h3 id="the-cssstyledeclaration-interface" data-level="4.1"><span class="secno">
   DOMString get(DOMString varName);
   boolean has(DOMString varName);
   void set(DOMString varName, DOMString varValue);
-  DOMString delete(DOMString varName);
+  boolean delete(DOMString varName);
 }
 </pre>
 <p>	The <a href="http://dev.w3.org/2006/webapi/WebIDL/#REPLACE-ME">map tuples</a>
@@ -717,7 +717,9 @@ <h3 id="the-cssstyledeclaration-interface" data-level="4.1"><span class="secno">
 			Invoke <code>removeProperty()</code>
 			on the <a data-autolink="link" href="#associated-style-declaration">associated style declaration</a>
 			by passing <var>varName</var> as its argument.
-			Return its returned value.
+			If the returned value is the empty string,
+			return <code>false</code>;
+			otherwise, return <code>true</code>.
 	</dd></dl>
 
 	<div class="example">

css-variables/Overview.src.html

@@ -635,7 +635,7 @@ <h3 id='the-CSSVariablesMap-interface'>
 		DOMString get(DOMString varName);
 		boolean has(DOMString varName);
 		void set(DOMString varName, DOMString varValue);
-		DOMString delete(DOMString varName);
+		boolean delete(DOMString varName);
 	}
 	</pre>
 
@@ -679,7 +679,9 @@ <h4 id='cssvariablesmap-methods'>
 			Invoke <code>removeProperty()</code>
 			on the <a>associated style declaration</a>
 			by passing <var>varName</var> as its argument.
-			Return its returned value.
+			If the returned value is the empty string,
+			return <code>false</code>;
+			otherwise, return <code>true</code>.
 	</dl>
 
 	<div class='example'>