🐛Report if defaultView is null in preconnect (ampproject#15993)

This is an attempt at fixing ampproject#15883.

The current suspicion is that the setTimeout call inside
`CustomElement#preconnect` delays the call to
`BaseElement#preconnectCallback`. During this delay, the parent window
is destroyed (maybe the iframe is removed from the DOM?), causing
`element.ownerDocument.defaultView` to return `null`.

It's frankly very surprising that timers would fire after a window is
destroyed, so we're only guarding `#preconnectCallback` for now. We may
later extend this to all timers if it fixes the bug.

Sidebar should not close when toolbar is present (ampproject#15990)

* Sidebar should not close  when toolbar is present

* Add jsdoc

Update whitelist (ampproject#15896)

Update whitelist (ampproject#15896)

SwG Release 0.1.22.13 (ampproject#15838)

Attempt to enable Safari 11.0 on SL (ampproject#15798)

* Attempt to enable Safari 11.0 on SL.

* Trigger tests on build system changes.

Fix amp-list w/o amp-bind with disable-faster-amp-list enabled. (ampp…

…roject#15722)

setImportantStyle (ampproject#15671)

Import SwG button stylesheet (ampproject#15566)

adding Amazon Publisher Services Callout URL (ampproject#15537)