Fixes for lightbox gallery and shadow docs. (#20712)

- Use AMP.win instead of global.
- Do the transition within the shadow doc, since that is where we are putting
  the styles for the transition.

extensions/amp-lightbox-gallery/0.1/amp-lightbox-gallery.js

@@ -959,6 +959,7 @@ export class AmpLightboxGallery extends AMP.BaseElement {
       // Prepare the actual image animation.
       imageAnimation = prepareImageAnimation({
         styleContainer: this.getAmpDoc().getHeadNode(),
+        transitionContainer: this.getAmpDoc().getBody(),
         srcImg,
         targetImg,
         srcImgRect: undefined,
@@ -1479,5 +1480,5 @@ function lightboxManagerForDoc(element) {
 AMP.extension(TAG, '0.1', AMP => {
   AMP.registerElement(TAG, AmpLightboxGallery, CSS);
   AMP.registerServiceForDoc('amp-lightbox-manager', LightboxManager);
-  Services.extensionsFor(global).addDocFactory(installLightboxGallery);
+  Services.extensionsFor(AMP.win).addDocFactory(installLightboxGallery);
 });