[asm.js] Fix [[Configurable]] attribute of exports.

R=clemensh@chromium.org
TEST=mjsunit/asm/asm-validation
BUG=chromium:715068,v8:5877

Change-Id: I26e0b84c94e5f036336f39e9d764f0588ff3ec0d
Reviewed-on: https://chromium-review.googlesource.com/486882Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44861}

src/wasm/wasm-module.cc

@@ -1865,6 +1865,7 @@ class InstantiationHelper {
     PropertyDescriptor desc;
     desc.set_writable(module_->is_asm_js());
     desc.set_enumerable(true);
+    desc.set_configurable(module_->is_asm_js());
 
     // Count up export indexes.
     int export_index = 0;

test/mjsunit/asm/asm-validation.js

@@ -474,6 +474,11 @@ function assertValidAsm(func) {
   assertFalse(o instanceof WebAssembly.Instance);
   assertTrue(o instanceof Object);
   assertTrue(o.__proto__ === Object.prototype);
+  var p = Object.getOwnPropertyDescriptor(o, "x")
+  assertTrue(p.writable);
+  assertTrue(p.enumerable);
+  assertTrue(p.configurable);
+  assertTrue(typeof o.x === 'function');
   o.x = 5;
   assertTrue(typeof o.x === 'number');
   assertTrue(o.__single_function__ === undefined);