Fix node missing ownerDocument issue in getComputedStyleForNode

In rare cases, DevTools frontend will call getComputedStyleForNode
on a node that no longer has an owner document. This CL fixes this case,
other it will result in a DCHECK failure in document.cc.

Change-Id: I33b023e9ee600294cb8a3e9ea833b48e60bc0827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421668Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Changhao Han <changhaohan@chromium.org>
Auto-Submit: Changhao Han <changhaohan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808852}

third_party/blink/renderer/core/inspector/inspector_css_agent.cc

@@ -1192,6 +1192,8 @@ Response InspectorCSSAgent::getComputedStyleForNode(
   response = dom_agent_->AssertNode(node_id, node);
   if (!response.IsSuccess())
     return response;
+  if (!node->ownerDocument())
+    return Response::ServerError("Node does not have an owner document");
 
   auto* computed_style_info =
       MakeGarbageCollected<CSSComputedStyleDeclaration>(node, true);