[cssom] Update IDL for [TreatNullAs] extended attribute

See whatwg/webidl#286

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=30107

Author: zcorpan

cssom/Overview.bs

@@ -503,7 +503,7 @@ An object that implements the <code>MediaList</code> interface has an associated
 <pre class=idl>
 [LegacyArrayClass]
 interface MediaList {
-  [TreatNullAs=EmptyString] stringifier attribute CSSOMString mediaText;
+  stringifier attribute [TreatNullAs=EmptyString] CSSOMString mediaText;
   readonly attribute unsigned long length;
   getter CSSOMString? item(unsigned long index);
   void appendMedium(CSSOMString medium);
@@ -2048,7 +2048,7 @@ interface CSSStyleDeclaration {
   [CEReactions] void setPropertyPriority(CSSOMString property, [TreatNullAs=EmptyString] CSSOMString priority);
   [CEReactions] CSSOMString removeProperty(CSSOMString property);
   readonly attribute CSSRule? parentRule;
-  [CEReactions, TreatNullAs=EmptyString] attribute CSSOMString cssFloat;
+  [CEReactions] attribute [TreatNullAs=EmptyString] CSSOMString cssFloat;
 };
 </pre>
 
@@ -2267,7 +2267,7 @@ is obtained by running the <a>CSS property to IDL attribute</a> algorithm for
 
 <pre class="idl extract">
 partial interface CSSStyleDeclaration {
-  [CEReactions, TreatNullAs=EmptyString] attribute CSSOMString _<var>camel_cased_attribute</var>;
+  [CEReactions] attribute [TreatNullAs=EmptyString] CSSOMString _<var>camel_cased_attribute</var>;
 };
 </pre>
 
@@ -2291,7 +2291,7 @@ algorithm for <var>property</var>, with the <i>lowercase first</i> flag set.
 
 <pre class="idl extract">
 partial interface CSSStyleDeclaration {
-  [CEReactions, TreatNullAs=EmptyString] attribute CSSOMString _<var>webkit_cased_attribute</var>;
+  [CEReactions] attribute [TreatNullAs=EmptyString] CSSOMString _<var>webkit_cased_attribute</var>;
 };
 </pre>
 
@@ -2316,7 +2316,7 @@ the following partial interface applies where <var>dashed attribute</var> is <va
 
 <pre class="idl extract">
 partial interface CSSStyleDeclaration {
-  [CEReactions, TreatNullAs=EmptyString] attribute CSSOMString _<var>dashed_attribute</var>;
+  [CEReactions] attribute [TreatNullAs=EmptyString] CSSOMString _<var>dashed_attribute</var>;
 };
 </pre>
 
@@ -2962,6 +2962,7 @@ Benjamin Poulain,
 Bj&ouml;rn H&ouml;hrmann,
 Boris Zbasky,
 Brian Kardell,
+Chris Dumez,
 Christian Krebs,
 Daniel Glazman,
 David Baron,