Deprecate the Element.prefix setter

Intent to Deprecate and Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/Tu9pL7gMPyo/f-eYf_AkpQMJ

The conclusion of the thread was to deprecate only at this time.

Review URL: https://codereview.chromium.org/301403002

git-svn-id: svn://svn.chromium.org/blink/trunk@175202 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/LayoutTests/fast/dom/Element/prefix-setter-exception-expected.txt

+CONSOLE WARNING: Setting 'Element.prefix' is deprecated, as it is read-only per DOM (http://dom.spec.whatwg.org/#element).
 Test for the implementation of DOM Level 3 Core API on Node Interface: prefix setter. INVALID_CHARACTER_ERR: Raised if the specified prefix contains an illegal character according to the XML version in use specified in the Document.xmlVersion attribute. http://www.w3.org/TR/DOM-Level-3-Core/core.html#ID-NodeNSPrefix
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

third_party/WebKit/LayoutTests/fast/dom/element-attribute-js-null-expected.txt

+CONSOLE WARNING: Setting 'Element.prefix' is deprecated, as it is read-only per DOM (http://dom.spec.whatwg.org/#element).
 CONSOLE WARNING: 'HTMLHeadElement.profile' is deprecated. The reflected attribute has no effect.
 This test setting various attributes of a elements to JavaScript null.
 

third_party/WebKit/LayoutTests/fast/dom/node-prefix-setter-namespace-exception-expected.txt

+CONSOLE WARNING: Setting 'Element.prefix' is deprecated, as it is read-only per DOM (http://dom.spec.whatwg.org/#element).
 Test how Node.prefix setter raises NAMESPACE_ERR.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

third_party/WebKit/Source/core/dom/Element.cpp

@@ -1201,7 +1201,7 @@ String Element::nodeName() const
 
 void Element::setPrefix(const AtomicString& prefix, ExceptionState& exceptionState)
 {
-    UseCounter::count(document(), UseCounter::ElementSetPrefix);
+    UseCounter::countDeprecation(document(), UseCounter::ElementSetPrefix);
 
     if (!prefix.isEmpty() && !Document::isValidName(prefix)) {
         exceptionState.throwDOMException(InvalidCharacterError, "The prefix '" + prefix + "' is not a valid name.");

third_party/WebKit/Source/core/frame/UseCounter.cpp

@@ -730,6 +730,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case HTMLHeadElementProfile:
         return "'HTMLHeadElement.profile' is deprecated. The reflected attribute has no effect.";
 
+    case ElementSetPrefix:
+        return "Setting 'Element.prefix' is deprecated, as it is read-only per DOM (http://dom.spec.whatwg.org/#element).";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();