Use tree positions for CreatePositionAt{Start,End}OfDocument

Narrator frequently calls ExpandToEnclosingUnit() for page or document
units when moving by words. This call was accounting for a sizable
fraction of each movement step, and about 56% of it was spent creating
the start and end positions. Switching to tree positions to find them
eliminates almost all of that cost.

Bug: 928948
Change-Id: I447aae5f089a27d041c2b173b16bf40629cb64ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1745547
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@{#686426}

ui/accessibility/ax_position.h

@@ -863,41 +863,29 @@ class AXPosition {
   }
 
   AXPositionInstance CreatePositionAtStartOfDocument() const {
-    if (IsNullPosition())
-      return CreateNullPosition();
-
-    AXPositionInstance iterator = Clone();
-    while (!iterator->IsNullPosition()) {
-      if (IsDocument(iterator->GetRole()) &&
-          iterator->CreateParentPosition()->IsNullPosition()) {
-        return iterator->CreatePositionAtStartOfAnchor();
-      }
-      iterator = iterator->CreateParentPosition();
+    AXPositionInstance position =
+        AsTreePosition()->CreateDocumentAncestorPosition();
+    if (!position->IsNullPosition()) {
+      position = position->CreatePositionAtStartOfAnchor();
+      if (IsTextPosition())
+        position = position->AsTextPosition();
     }
-    return CreateNullPosition();
+    return position;
   }
 
   AXPositionInstance CreatePositionAtEndOfDocument() const {
-    if (IsNullPosition())
-      return CreateNullPosition();
-
-    AXPositionInstance iterator = Clone();
-    while (!iterator->IsNullPosition()) {
-      if (IsDocument(iterator->GetRole()) &&
-          iterator->CreateParentPosition()->IsNullPosition()) {
-        AXPositionInstance tree_position = iterator->AsTreePosition();
-        DCHECK(tree_position);
-        while (tree_position->AnchorChildCount()) {
-          tree_position = tree_position->CreateChildPositionAt(
-              tree_position->AnchorChildCount() - 1);
-        }
-        iterator =
-            tree_position->AsLeafTextPosition()->CreatePositionAtEndOfAnchor();
-        return iterator;
+    AXPositionInstance position =
+        AsTreePosition()->CreateDocumentAncestorPosition();
+    if (!position->IsNullPosition()) {
+      while (position->AnchorChildCount()) {
+        position =
+            position->CreateChildPositionAt(position->AnchorChildCount() - 1);
       }
-      iterator = iterator->CreateParentPosition();
+      position = position->CreatePositionAtEndOfAnchor();
+      if (IsTextPosition())
+        position = position->AsTextPosition();
     }
-    return CreateNullPosition();
+    return position;
   }
 
   AXPositionInstance CreateChildPositionAt(int child_index) const {
@@ -2382,6 +2370,18 @@ class AXPosition {
     return false;
   }
 
+  AXPositionInstance CreateDocumentAncestorPosition() const {
+    AXPositionInstance iterator = Clone();
+    while (!iterator->IsNullPosition()) {
+      if (IsDocument(iterator->GetRole()) &&
+          iterator->CreateParentPosition()->IsNullPosition()) {
+        break;
+      }
+      iterator = iterator->CreateParentPosition();
+    }
+    return iterator;
+  }
+
   AXPositionKind kind_;
   AXTreeID tree_id_;
   int32_t anchor_id_;