[es6] Set correct length for Reflect.get (should be 2, not 3).

R=rossberg
BUG=

Review URL: https://codereview.chromium.org/1498983003

Cr-Commit-Position: refs/heads/master@{#32615}

src/bootstrapper.cc

@@ -2044,7 +2044,7 @@ void Genesis::InitializeGlobal_harmony_reflect() {
   SimpleInstallFunction(reflect, factory->deleteProperty_string(),
                         Builtins::kReflectDeleteProperty, 2, true);
   SimpleInstallFunction(reflect, factory->get_string(),
-                        Builtins::kReflectGet, 3, false);
+                        Builtins::kReflectGet, 2, false);
   SimpleInstallFunction(reflect, factory->getOwnPropertyDescriptor_string(),
                         Builtins::kReflectGetOwnPropertyDescriptor, 2, true);
   SimpleInstallFunction(reflect, factory->getPrototypeOf_string(),

test/mjsunit/harmony/reflect.js

@@ -71,7 +71,7 @@ function prepare(target) {
 
 
 (function testReflectGetArity() {
-  assertEquals(3, Reflect.get.length);
+  assertEquals(2, Reflect.get.length);
 })();