Remove [TreatNullAs=NullString] in CSSStyleDeclaration

Replace NullString with EmptyString since [TreatNullAs=NullString] is
deprecated.

This CL has no behavior changes.

Bug: 497307, 831544
Change-Id: I4ae886b3f3b1159f67b9897e2e0f4f0423477f7f
Reviewed-on: https://chromium-review.googlesource.com/c/1175589Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Amos Lim <eui-sang.lim@samsung.com>
Cr-Commit-Position: refs/heads/master@{#596134}

third_party/blink/renderer/core/css/css_style_declaration.idl

@@ -39,7 +39,5 @@
     // https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-camel-cased-attribute
     // https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-dashed-attribute
     [Affects=Nothing] getter DOMString (DOMString name);
-    // TODO(crbug.com/831544): [TreatNullAs=EmptyString] should be used instead
-    // of [TreatNullAs=NullString].
-    [CEReactions, CallWith=ScriptState] setter void (DOMString property, [TreatNullAs=NullString] DOMString propertyValue);
+    [CEReactions, CallWith=ScriptState] setter void (DOMString property, [TreatNullAs=EmptyString] DOMString propertyValue);
 };