Preserve images within figure elements.

This CL fixes an issue where images nested within `<div>` elements
inside a `<figure>` tag were being incorrectly removed by Readability's
cleaning process. Specifically, a structure like
`<figure><div><div><img></div></div></figure>` would first be
transformed to `<figure><div><p><img></p></div></figure>`, and then
the outer `<div>` (and its contents) would be erroneously identified
as extraneous and removed.

The fix introduces a targeted exception within _cleanConditionally().
It prevents the removal of `<div>` elements that meet the following
criteria:
* The element is a `<div>`.
* The `<div>` is an ancestor of a `<figure>` element.
* The `<div>` contains a single `<img>` element (potentially nested).

Author: samuelhuang

Readability.js

@@ -2506,6 +2506,11 @@ Readability.prototype = {
         return false;
       }
 
+      // Handle <img> buried inside nested <div> layers in <figure>.
+      if (tag === "div" && this._hasAncestorTag(node, "figure") && this._isSingleImage(node)) {
+        return false;
+      }
+
       var weight = this._getClassWeight(node);
 
       this.log("Cleaning Conditionally", node);