DevTools: [PrettyPrint] support arrow function

BUG=579770
R=pfeldman, dgozman

Review URL: https://codereview.chromium.org/1614563002

Cr-Commit-Position: refs/heads/master@{#370773}

third_party/WebKit/LayoutTests/inspector/sources/pretty-print-javascript-6-expected.txt

@@ -80,3 +80,16 @@ Correct mapping for <function>
 Correct mapping for <alert>
 Correct mapping for <2000>
 
+Running: arrowFunction
+====== 8< ------
+function test(arg) {
+    console.log(arg);
+}
+test(a=>a + 2);
+
+------ >8 ======
+Correct mapping for <function>
+Correct mapping for <console>
+Correct mapping for <=>>
+Correct mapping for <2>
+

third_party/WebKit/LayoutTests/inspector/sources/pretty-print-javascript-6.html

@@ -46,6 +46,12 @@ function test()
         {
             var mappingQueries = ["setTimeout", "function", "alert", "2000"];
             testJSFormatter("setTimeout(function(){alert(1);},2000);", mappingQueries, next);
+        },
+
+        function arrowFunction(next)
+        {
+            var mappingQueries = ["function", "console", "=>", "2"];
+            testJSFormatter("function test(arg){console.log(arg);}test(a=>a+2);", mappingQueries, next);
         }
     ]);
 }

third_party/WebKit/Source/devtools/front_end/script_formatter_worker/ESTreeWalker.js

@@ -75,6 +75,7 @@ FormatterWorker.ESTreeWalker.prototype = {
 /** @enum {!Array.<string>} */
 FormatterWorker.ESTreeWalker._walkOrder = {
     "ArrayExpression": ["elements"],
+    "ArrowFunctionExpression": ["params", "body"],
     "AssignmentExpression": ["left", "right"],
     "BinaryExpression": ["left", "right"],
     "BlockStatement": ["body"],