[css-typed-om] replace undefined variable with defined variable

there is using undefined variable in iterable.html.
it is cause to fail layout test.

Bug: 774887
Change-Id: Ibe516bcd8a24cc21ed604a1a9cf85e392a1c40c5
Reviewed-on: https://chromium-review.googlesource.com/828562Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Cr-Commit-Position: refs/heads/master@{#524634}

third_party/WebKit/LayoutTests/typedcssom/the-stylepropertymap/iterable-expected.txt

-This is a testharness.js-based test.
-PASS Iterating over an empty StylePropertyMap gives a zero-length array
-PASS StylePropertyMap iterates properties in correct order
-FAIL StylePropertyMap iterator returns CSS properties with the correct CSSStyleValue result is not defined
-PASS StylePropertyMap iterator returns list-valued properties with the correct CSSStyleValue
-PASS StylePropertyMap iterator returns custom properties with the correct CSSStyleValue
-Harness: the test ran to completion.
-

third_party/WebKit/LayoutTests/typedcssom/the-stylepropertymap/iterable.html

@@ -25,7 +25,7 @@ test(() => {
 
 test(() => {
   const styleMap = document.getElementById('target-multiple').attributeStyleMap;
-  const [key, value] = [...styleMap][2];
+  const [key, result] = [...styleMap][2];
   assert_equals(key, 'width');
   assert_style_value_equals(result, CSS.px(10));
 }, 'StylePropertyMap iterator returns CSS properties with the correct CSSStyleValue');