[js-perf-test] add microbenchmarks for String Iterators

test/js-perf-test/JSTests.json

 {
   "name": "JSTests",
   "run_count": 5,
   "run_count_android_arm": 3,
   "run_count_android_arm64": 3,
   "timeout": 120,
   "units": "score",
   "total": true,
   "resources": ["base.js"],
   "tests": [
@@ skipping 106 matching lines @@
       "name": "Strings",
       "path": ["Strings"],
       "main": "run.js",
       "resources": ["harmony-string.js"],
       "results_regexp": "^%s\\-Strings\\(Score\\): (.+)$",
       "tests": [
         {"name": "StringFunctions"}
       ]
     },
     {
+      "name": "StringIterators",
+      "path": ["StringIterators"],
+      "main": "run.js",
+      "resources": ["string-iterator.js"],
+      "results_regexp": "^%s\\-StringIterators\\(Score\\): (.+)$",
+      "tests": [
+        {"name": "Spread_OneByteShort"},
+        {"name": "Spread_TwoByteShort"},
+        {"name": "Spread_WithSurrogatePairsShort"},
+        {"name": "ForOf_OneByteShort"},
+        {"name": "ForOf_TwoByteShort"},
+        {"name": "ForOf_WithSurrogatePairsShort"},
+        {"name": "ForOf_OneByteLong"},
+        {"name": "ForOf_TwoByteLong"},
+        {"name": "ForOf_WithSurrogatePairsLong"}
+      ]
+    },
+    {
       "name": "Templates",
       "path": ["Templates"],
       "main": "run.js",
       "resources": ["templates.js"],
       "run_count": 5,
       "units": "score",
       "results_regexp": "^%s\\-Templates\\(Score\\): (.+)$",
       "total": true,
       "tests": [
         {"name": "Untagged"},
@@ skipping 77 matching lines @@
         {"name": "Object.hasOwnProperty--DEINTERN-prop"},
         {"name": "Object.hasOwnProperty--NE-INTERN-prop"},
         {"name": "Object.hasOwnProperty--NE-DEINTERN-prop"},
         {"name": "Object.hasOwnProperty--el"},
         {"name": "Object.hasOwnProperty--el-str"},
         {"name": "Object.hasOwnProperty--NE-el"}
       ]
     }
   ]
 }