Issue #1643 - Part 1: Add GetNodeDepth() to nsContentUtils.

dom/base/nsContentUtils.cpp

@@ -9843,3 +9843,17 @@ nsContentUtils::GetClosestNonNativeAnonymousAncestor(Element* aElement)
   }
   return e;
 }
+
+/* static */ uint32_t
+nsContentUtils::GetNodeDepth(nsINode* aNode)
+{
+  uint32_t depth = 1;
+
+  MOZ_ASSERT(aNode, "Node shouldn't be null");
+
+  while ((aNode = aNode->GetParentNode())) {
+    ++depth;
+  }
+
+  return depth;
+}

dom/base/nsContentUtils.h

@@ -2763,6 +2763,14 @@ public:
   static bool
   IsCustomElementsEnabled() { return sIsCustomElementsEnabled; }
 
+  /**
+   * Returns the length of the parent-traversal path (in terms of the number of
+   * nodes) to an unparented/root node from aNode. An unparented/root node is
+   * considered to have a depth of 1, its children have a depth of 2, etc.
+   * aNode is expected to be non-null.
+   */
+  static uint32_t GetNodeDepth(nsINode* aNode);
+
 private:
   static bool InitializeEventTable();