Bug 1404789 - Stop reconstructing frames for the whole shadow root each time content is inserted in a shadow tree

* Cleanup a bit the ShadowRoot code
* Privatize ShadowRoot methods
* When the shadow tree distribution changes, post a restyle + reframe
* Simplify ShadowRoot::IsPooledNode
* Be a bit better at detecting distribution changes

Tag #1375

layout/base/crashtests/1404789-1.html

@@ -0,0 +1,16 @@
+<!doctype html>
+<script>
+  // Test for content redistribution outside of the document.
+  // Passes if it doesn't assert.
+  let host = document.createElement('div');
+  host.innerHTML = "<div id='foo'></div>";
+
+  let shadowRoot = host.createShadowRoot();
+  shadowRoot.innerHTML = "<content select='#foo'></content>";
+
+  host.firstElementChild.removeAttribute('id');
+
+  // Move to the document, do the same.
+  document.documentElement.appendChild(host);
+  host.firstElementChild.setAttribute('id', 'foo');
+</script>