Performance improvement for AXPosition<>::AsTreePosition

While investigating opportunities where AXRange<>::{begin|end}() can
be improved, I found that AsTreePosition was one of the higher cost
methods.

This is because it's called many times in order to create an
AXRange<>::Iterator. Both AXRange<>::AsForwardRange and the Iterator
constructor via AXPosition<>::AsLeafTextPosition call AsTreePosition.
The reason it's expensive is because of AXPosition<>::MaxTextOffset.

For the scenario I was measuring, this CL reduced the overall amount
of time spent in AXRange::begin by ~30-35%, and the overall amount of
time spent in AXPosition<>::AsTreePosition by ~50%.

I crafted a document where there are many inline nodes that form
sentences with various levels of nesting, and used Narrator with
UIA enabled to navigate 5 sentences.

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

Before :
  * Weight for callees of AXRange<>::begin : 7889
  * Weight for callees of AXPosition<>::AsTreePosition : 20418

After :
  * Weight for callees of AXRange<>::begin : 5246
  * Weight for callees of AXPosition<>::AsTreePosition : 10298

Bug: 928948
Change-Id: I24be1be302b9e22bc85412f091b5f464274f1dfb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824064
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702075}

ui/accessibility/ax_position.h

@@ -707,11 +707,14 @@ class AXPosition {
     AXPositionInstance copy = Clone();
     DCHECK(copy);
     DCHECK_GE(copy->text_offset_, 0);
-    if (!copy->AnchorChildCount() &&
-        copy->text_offset_ != copy->MaxTextOffset()) {
-      copy->child_index_ = BEFORE_TEXT;
-    } else {
-      copy->child_index_ = 0;
+    if (!copy->AnchorChildCount()) {
+      const int max_text_offset = copy->MaxTextOffset();
+      copy->child_index_ =
+          (max_text_offset != 0 && copy->text_offset_ != max_text_offset)
+              ? BEFORE_TEXT
+              : 0;
+      copy->kind_ = AXPositionKind::TREE_POSITION;
+      return copy;
     }
 
     // Blink doesn't always remove all deleted whitespace at the end of a
@@ -720,23 +723,22 @@ class AXPosition {
     // last child that we can reach with the current text offset and ignore any
     // remaining children.
     int current_offset = 0;
-    for (int i = 0; i < copy->AnchorChildCount(); ++i) {
-      AXPositionInstance child = copy->CreateChildPositionAt(i);
+    int child_index = 0;
+    for (; child_index < copy->AnchorChildCount(); ++child_index) {
+      AXPositionInstance child = copy->CreateChildPositionAt(child_index);
       DCHECK(child);
       int child_length = child->MaxTextOffsetInParent();
       if (copy->text_offset_ >= current_offset &&
           (copy->text_offset_ < (current_offset + child_length) ||
            (copy->affinity_ == ax::mojom::TextAffinity::kUpstream &&
             copy->text_offset_ == (current_offset + child_length)))) {
-        copy->child_index_ = i;
         break;
       }
 
       current_offset += child_length;
     }
-    if (current_offset >= copy->MaxTextOffset())
-      copy->child_index_ = copy->AnchorChildCount();
 
+    copy->child_index_ = child_index;
     copy->kind_ = AXPositionKind::TREE_POSITION;
     return copy;
   }