Performance improvement in UpdateComputedHypertext

AXPlatformNodeBase::UpdateComputedHypertext was using an expensive path
to navigate unignored children. Replacing use of |ChildAtIndex| with
|GetFirstChild| and |GetNextSibling| when iterating unignored children.

Now instead of having to walk over all children up to the Nth unignored
child for each iteration, it will find the first unignored child, then
chain finding each unignored sibling.

For the scenario I was measuring, this CL reduced the overall amount
of time spent in AXTree::Unserialize by ~77%, and time spent in
AXPlatformNodeBase::UpdateComputedHypertext by ~90%!

I crafted a document where loading the document resulted in Unserialize
having to process ~40k AXNodeData items in one AXTreeUpdate.

I used "UI for ETW" to capture perf traces, and WPA to view the data.

Before :
  * Weight in callstack of AXTree::Unserialize : 6424
  * Weight in callstack of UpdateComputedHypertext : 5449

After :
  * Weight in callstack of AXTree::Unserialize : 1457
  * Weight in callstack of UpdateComputedHypertext : 447

Bug: 1017829
Change-Id: I0edd29f4bba4811942a96c4f65fa96401dd10cea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879767Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709956}

ui/accessibility/platform/ax_platform_node_base.cc

@@ -1139,9 +1139,7 @@ void AXPlatformNodeBase::UpdateComputedHypertext() {
     return;
   }
 
-  int child_count = GetChildCount();
-
-  if (!child_count) {
+  if (!GetChildCount()) {
     if (IsRichTextField()) {
       // We don't want to expose any associated label in IA2 Hypertext.
       return;
@@ -1157,10 +1155,8 @@ void AXPlatformNodeBase::UpdateComputedHypertext() {
   // the character index of each embedded object character to the id of the
   // child object it points to.
   base::string16 hypertext;
-  for (int i = 0; i < child_count; ++i) {
-    const auto* child = FromNativeViewAccessible(ChildAtIndex(i));
-
-    DCHECK(child);
+  for (AXPlatformNodeBase* child = GetFirstChild(); child;
+       child = child->GetNextSibling()) {
     // Similar to Firefox, we don't expose text-only objects in IA2 hypertext.
     if (child->IsTextOnlyObject()) {
       hypertext_.hypertext +=