Fix flaky DumpAccessibilityTreeTest tests

This change fixes flaky test failures caused by the following CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1591484/22

Flaky behavior is fixed for the following tests:
-DumpAccessibilityTreeTest.AccessibilityIframePostEnable
-DumpAccessibilityTreeTest.AccessibilityIframe

DumpAccessibilityTreeTest.AccessibilityIframePostEnable is also
re-enabled as a revert to the following change:
https://chromium-review.googlesource.com/c/chromium/src/+/1638201

(cherry picked from commit 419aea493217e43ace35f6b0b5c40433592c8d45)

Bug: 969185,972562
Change-Id: I1c961f215936a04ba54b5dd5ca6ba35205567648
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1638925
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Alison Maher <almaher@microsoft.com>
Cr-Original-Commit-Position: refs/heads/master@{#665944}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1650616
Cr-Commit-Position: refs/branch-heads/3809@{#223}
Cr-Branched-From: d82dec1a818f378c464ba307ddd9c92133eac355-refs/heads/master@{#665002}

diff --git a/content/browser/accessibility/browser_accessibility.cc b/content/browser/accessibility/browser_accessibility.cc
index 044c458d..7975e77 100644
--- a/content/browser/accessibility/browser_accessibility.cc
+++ b/content/browser/accessibility/browser_accessibility.cc

@@ -68,8 +68,7 @@
 }
 
 bool BrowserAccessibility::PlatformIsLeaf() const {
-  if (!HasStringAttribute(ax::mojom::StringAttribute::kChildTreeId) &&
-      InternalChildCount() == 0)
+  if (InternalChildCount() == 0)
     return true;
 
   // These types of objects may have children that we use as internal