Add a UseCounter for Text.wholeText

Text.replaceWholeText() was removed from DOM and has very low usage:
https://www.chromestatus.com/metrics/feature/timeline/popularity/100

Measure Text.wholeText also to see if it is similarly unused.

Discussed briefly with Anne van Kesteren on IRC:
http://krijnhoetmer.nl/irc-logs/whatwg/20141117#l-248

Review URL: https://codereview.chromium.org/730003003

git-svn-id: svn://svn.chromium.org/blink/trunk@185453 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/dom/Text.idl

@@ -25,7 +25,7 @@
     [RaisesException] Text splitText(unsigned long offset);
 
     // Introduced in DOM Level 3:
-    readonly attribute DOMString       wholeText;
+    [MeasureAs=TextWholeText] readonly attribute DOMString wholeText;
     [MeasureAs=TextReplaceWholeText] Text replaceWholeText(DOMString content); // Removed from DOM4.
 
     // Shadow DOM API

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

@@ -557,6 +557,7 @@ public:
         UIEventCharCode = 596,
         UIEventKeyCode = 597,
         UIEventWhich = 598,
+        TextWholeText = 599,
 
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.