Mark LayoutTest fast/js/kde/StringObject.html for manual rebaseline.

This is necessary since a ES6 spec violation has been fixed in V8.

BUG=401408
TBR=yangguo@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/447213003

git-svn-id: svn://svn.chromium.org/blink/trunk@179708 bbb929c8-8fbe-4397-9dbb-9b2b20218538