Remove adopted stylesheets when adopting to a different document

When adopting a subtree to a different document, we'll remove all the
adopted stylesheets in that subtree, instead of keeping them and then
ignoring them in style calculation.

Relevant discussion: WICG/construct-stylesheets#23

Bug: 807560
Change-Id: I7ea6c869892cabd9d3b5a765f26364e51b1419b4
Reviewed-on: https://chromium-review.googlesource.com/c/1415068
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623660}

css/cssom/CSSStyleSheet-constructable.html

@@ -501,15 +501,18 @@
   span.shadowRoot.adoptedStyleSheets = [sheet];
   assert_equals(getComputedStyle(shadowDiv).color, "rgb(255, 0, 0)");
 
+  document.adoptNode(span);
+  assert_equals(span.shadowRoot.adoptedStyleSheets.length, 1);
+  assert_equals(span.shadowRoot.adoptedStyleSheets[0], sheet);
+
   const iframe = document.createElement("iframe");
   document.body.appendChild(iframe);
   iframe.contentDocument.adoptNode(span);
   iframe.contentDocument.body.appendChild(span);
   assert_not_equals(span.shadowRoot, null);
-  assert_equals(span.shadowRoot.adoptedStyleSheets.length, 1);
-  assert_equals(span.shadowRoot.adoptedStyleSheets[0], sheet);
+  assert_equals(span.shadowRoot.adoptedStyleSheets.length, 0);
   assert_equals(getComputedStyle(shadowDiv).color, "rgb(0, 0, 0)");
-}, 'Adopting a shadow host will move adoptedStyleSheets but it is not applied');
+}, 'Adopting a shadow host will empty adoptedStyleSheets if adopting to a different document');
 
 </script>