Add fromCSSValue methods to LengthValue subclasses, remove extra method in CSSSimpleLength.

third_party/WebKit/Source/core/css/cssom/CSSSimpleLength.idl

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
     Constructor(double value, DOMString type),
     Exposed=(Window,PaintWorklet),
     RaisesException=Constructor,
     RuntimeEnabled=CSSTypedOM
 ] interface CSSSimpleLength : CSSLengthValue {
-    [EnforceRange] attribute double value;
+    [EnforceRange] readonly attribute double value;
     [ImplementedAs=unit] readonly attribute LengthType type;
 };