Avoid traversing the FrameTree up in FrameTree::top()

https://bugs.webkit.org/show_bug.cgi?id=185394

Reviewed by Chris Dumez.

Credits & Patch Authored by Chris Dumez.

Avoid traversing the FrameTree up in FrameTree::top() and rely on Frame::mainFrame() instead to get
the answer right away. There is a potential behavior change if calling FrameTree::top() on a detached
frame (it will keep return the main frame now instead of returning the frame itself). However, it
should not matter in practice as we should not be dealing with the FrameTree of a detached frame.

* Source/WebCore/page/FrameTree.cpp:
(FrameTree::top):

Canonical link: https://commits.webkit.org/273230@main

Source/WebCore/page/FrameTree.cpp

@@ -516,10 +516,8 @@ Frame* FrameTree::deepLastChild() const
 
 Frame& FrameTree::top() const
 {
-    auto* frame = &m_thisFrame;
-    for (auto* parent = &m_thisFrame; parent; parent = parent->tree().parent())
-        frame = parent;
-    return *frame;
+    ASSERT(m_thisFrame.isMainFrame() || m_thisFrame.tree().parent());
+    return m_thisFrame.mainFrame();
 }
 
 unsigned FrameTree::depth() const