Test assignment to indexed window properties

The actual bug here was fixed in V8, but we should have a test at the Blink
layer too.

R=adamk
BUG=257748

Review URL: https://chromiumcodereview.appspot.com/19923006

git-svn-id: svn://svn.chromium.org/blink/trunk@154714 bbb929c8-8fbe-4397-9dbb-9b2b20218538