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

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

content/browser/accessibility/browser_accessibility.cc

@@ -68,8 +68,7 @@ void BrowserAccessibility::Init(BrowserAccessibilityManager* manager,
 }
 
 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

content/browser/accessibility/dump_accessibility_tree_browsertest.cc

@@ -1305,19 +1305,11 @@ IN_PROC_BROWSER_TEST_P(DumpAccessibilityTreeTest, AccessibilityIframe) {
   RunHtmlTest(FILE_PATH_LITERAL("iframe.html"));
 }
 
-#if defined(OS_WIN)
-IN_PROC_BROWSER_TEST_P(DumpAccessibilityTreeTest,
-                       DISABLED_AccessibilityIframePostEnable) {
-  enable_accessibility_after_navigating_ = true;
-  RunHtmlTest(FILE_PATH_LITERAL("iframe.html"));
-}
-#else
 IN_PROC_BROWSER_TEST_P(DumpAccessibilityTreeTest,
                        AccessibilityIframePostEnable) {
   enable_accessibility_after_navigating_ = true;
   RunHtmlTest(FILE_PATH_LITERAL("iframe.html"));
 }
-#endif
 
 IN_PROC_BROWSER_TEST_P(DumpAccessibilityTreeTest,
                        AccessibilityIframeCrossProcess) {