Ensure <img> elements converted to <amp-img> which have layout=intris…

…inc also get object-fit:contain

This guards against image stretching.

See <ampproject/amphtml#21371 (comment)>.
Props @cathyxz

assets/css/amp-default.css

@@ -1,6 +1,11 @@
-.amp-wp-unknown-size [src] {
-	/** Worst case scenario when we can't figure out dimensions for an image. **/
-	/** Force the image into a box of fixed dimensions and use object-fit to scale. **/
+
+/*
+ * Prevent cases of amp-img converted from img to appear with stretching by using object-fit to scale.
+ * See <https://github.com/ampproject/amphtml/issues/21371#issuecomment-475443219>.
+ * Also use object-fit:contain in worst case scenario when we can't figure out dimensions for an image.
+ */
+amp-img.amp-wp-enforced-sizes[layout=intrinsic] > img,
+.amp-wp-unknown-size > img {
 	object-fit: contain;
 }