Fix tests failure with Chrome 71

[zhouyx]

[dreamofabear]

Fixed the Travis failures in #19658.

Saucelabs just upgraded to 71 too and different (integration) tests are failing: https://travis-ci.org/ampproject/amphtml/jobs/464777879

[dreamofabear]

Looks like they're mostly amp-form integration tests. I'll fix the single amp-bind one.

/to @cvializ