Low-hanging optimization for AXPosition navigation

|LowestCommonAnchor| is always cheaper than |LowestCommonAncestor|
when the ancestor position data is not needed. I made some
low-hanging optimizations to AXPosition after adding
|CreateAncestorPosition|, since in many cases the position was
being discarded after calling |LowestCommonAncestor|.

Note that |LowestCommonAncestor| is equivalent to calling
|CreateAncestorPosition| with the result of |LowestCommonAnchor|,
so this is an improvement whenever |LowestCommonAnchor| is not the
same as the anchor of the initial position.
In any other case, there will be no difference in performance.

The methods that took this optimization are :
* CreateNextWordStartPosition
* CreatePreviousWordStartPosition
* CreateNextWordEndPosition
* CreatePreviousWordEndPosition
* CreatePreviousFormatStartPosition
* CreateNextFormatEndPosition
* CreateBoundaryStartPosition
* CreateBoundaryEndPosition

Bug: 928948
Change-Id: Ie25f5dfb293c604e0c34f348d276edac14d35ad3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814670
Commit-Queue: Adam Ettenberger <adettenb@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#699468}

ui/accessibility/ax_position.h

@@ -1439,10 +1439,9 @@ class AXPosition {
     // If the word boundary is in the same subtree, return a position rooted at
     // this position's anchor. This is necessary because we don't want to return
     // a position that might be in the shadow DOM when this position is not.
-    AXPositionInstance common_ancestor =
-        text_position->LowestCommonAncestor(*this);
-    if (GetAnchor() == common_ancestor->GetAnchor()) {
-      text_position = std::move(common_ancestor);
+    const AXNodeType* common_anchor = text_position->LowestCommonAnchor(*this);
+    if (GetAnchor() == common_anchor) {
+      text_position = text_position->CreateAncestorPosition(common_anchor);
     } else if (boundary_behavior == AXBoundaryBehavior::StopAtAnchorBoundary) {
       return CreatePositionAtEndOfAnchor();
     }
@@ -1521,10 +1520,9 @@ class AXPosition {
     // If the word boundary is in the same subtree, return a position rooted at
     // this position's anchor. This is necessary because we don't want to return
     // a position that might be in the shadow DOM when this position is not.
-    AXPositionInstance common_ancestor =
-        text_position->LowestCommonAncestor(*this);
-    if (GetAnchor() == common_ancestor->GetAnchor()) {
-      text_position = std::move(common_ancestor);
+    const AXNodeType* common_anchor = text_position->LowestCommonAnchor(*this);
+    if (GetAnchor() == common_anchor) {
+      text_position = text_position->CreateAncestorPosition(common_anchor);
     } else if (boundary_behavior == AXBoundaryBehavior::StopAtAnchorBoundary) {
       return CreatePositionAtStartOfAnchor();
     }
@@ -1609,10 +1607,9 @@ class AXPosition {
     // If the word boundary is in the same subtree, return a position rooted at
     // this position's anchor. This is necessary because we don't want to return
     // a position that might be in the shadow DOM when this position is not.
-    AXPositionInstance common_ancestor =
-        text_position->LowestCommonAncestor(*this);
-    if (GetAnchor() == common_ancestor->GetAnchor()) {
-      text_position = std::move(common_ancestor);
+    const AXNodeType* common_anchor = text_position->LowestCommonAnchor(*this);
+    if (GetAnchor() == common_anchor) {
+      text_position = text_position->CreateAncestorPosition(common_anchor);
     } else if (boundary_behavior == AXBoundaryBehavior::StopAtAnchorBoundary) {
       return CreatePositionAtEndOfAnchor();
     }
@@ -1697,10 +1694,9 @@ class AXPosition {
     // If the word boundary is in the same subtree, return a position rooted at
     // this position's anchor. This is necessary because we don't want to return
     // a position that might be in the shadow DOM when this position is not.
-    AXPositionInstance common_ancestor =
-        text_position->LowestCommonAncestor(*this);
-    if (GetAnchor() == common_ancestor->GetAnchor()) {
-      text_position = std::move(common_ancestor);
+    const AXNodeType* common_anchor = text_position->LowestCommonAnchor(*this);
+    if (GetAnchor() == common_anchor) {
+      text_position = text_position->CreateAncestorPosition(common_anchor);
     } else if (boundary_behavior == AXBoundaryBehavior::StopAtAnchorBoundary) {
       return CreatePositionAtStartOfAnchor();
     }
@@ -1798,10 +1794,9 @@ class AXPosition {
     // at the current position.
     // This is necessary because we don't want to return any position that might
     // be in the shadow DOM if the original position was not.
-    AXPositionInstance common_ancestor =
-        tree_position->LowestCommonAncestor(*this);
-    if (GetAnchor() == common_ancestor->GetAnchor()) {
-      tree_position = std::move(common_ancestor);
+    const AXNodeType* common_anchor = tree_position->LowestCommonAnchor(*this);
+    if (GetAnchor() == common_anchor) {
+      tree_position = tree_position->CreateAncestorPosition(common_anchor);
     } else if (boundary_behavior == AXBoundaryBehavior::StopAtAnchorBoundary) {
       return CreatePositionAtStartOfAnchor();
     }
@@ -1865,10 +1860,9 @@ class AXPosition {
     // rooted at the current position.
     // This is necessary because we don't want to return any position that might
     // be in the shadow DOM if the original position was not.
-    AXPositionInstance common_ancestor =
-        tree_position->LowestCommonAncestor(*this);
-    if (GetAnchor() == common_ancestor->GetAnchor()) {
-      tree_position = std::move(common_ancestor);
+    const AXNodeType* common_anchor = tree_position->LowestCommonAnchor(*this);
+    if (GetAnchor() == common_anchor) {
+      tree_position = tree_position->CreateAncestorPosition(common_anchor);
     } else if (boundary_behavior == AXBoundaryBehavior::StopAtAnchorBoundary) {
       return CreatePositionAtEndOfAnchor();
     }
@@ -2052,10 +2046,9 @@ class AXPosition {
     // If the boundary is in the same subtree, return a position rooted at this
     // position's anchor. This is necessary because we don't want to return a
     // position that might be in the shadow DOM when this position is not.
-    AXPositionInstance common_ancestor =
-        text_position->LowestCommonAncestor(*this);
-    if (GetAnchor() == common_ancestor->GetAnchor()) {
-      text_position = std::move(common_ancestor);
+    const AXNodeType* common_anchor = text_position->LowestCommonAnchor(*this);
+    if (GetAnchor() == common_anchor) {
+      text_position = text_position->CreateAncestorPosition(common_anchor);
     } else if (boundary_behavior == AXBoundaryBehavior::StopAtAnchorBoundary) {
       return (boundary_direction == AXTextBoundaryDirection::kForwards)
                  ? CreatePositionAtEndOfAnchor()
@@ -2129,10 +2122,9 @@ class AXPosition {
     // If the boundary is in the same subtree, return a position rooted at this
     // position's anchor. This is necessary because we don't want to return a
     // position that might be in the shadow DOM when this position is not.
-    AXPositionInstance common_ancestor =
-        text_position->LowestCommonAncestor(*this);
-    if (GetAnchor() == common_ancestor->GetAnchor()) {
-      text_position = std::move(common_ancestor);
+    const AXNodeType* common_anchor = text_position->LowestCommonAnchor(*this);
+    if (GetAnchor() == common_anchor) {
+      text_position = text_position->CreateAncestorPosition(common_anchor);
     } else if (boundary_behavior == AXBoundaryBehavior::StopAtAnchorBoundary) {
       return (boundary_direction == AXTextBoundaryDirection::kForwards)
                  ? CreatePositionAtEndOfAnchor()