Low-hanging optimization for ITextRangeProvider::ExpandToEnclosingUnit

|LowestCommonAnchor| is always cheaper than |LowestCommonAncestor|
when the ancestor position data is not needed.

AXPlatformNodeTextRangeProviderWin::ExpandToEnclosingUnit for
TextUnit_Page only needs the |LowestCommonAnchor| to know the
result of |AXNode::OwnerTree()->HasPaginationSupport|.

Bug: 928948
Change-Id: Ida1926da0c1d54b2a9b57abd1867862daa4b4984
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815350
Commit-Queue: Adam Ettenberger <adettenb@microsoft.com>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#699032}

ui/accessibility/platform/ax_platform_node_textrangeprovider_win.cc

@@ -177,8 +177,8 @@ STDMETHODIMP AXPlatformNodeTextRangeProviderWin::ExpandToEnclosingUnit(
     case TextUnit_Page: {
       // If the document doesn't support pagination, default to document units
       // per UIA spec
-      AXPositionInstance common_ancestor = start_->LowestCommonAncestor(*end_);
-      if (common_ancestor->GetAnchor()->tree()->HasPaginationSupport()) {
+      const AXNode* common_anchor = start_->LowestCommonAnchor(*end_);
+      if (common_anchor->tree()->HasPaginationSupport()) {
         start_ = start_->CreatePreviousPageStartPosition(
             ui::AXBoundaryBehavior::StopIfAlreadyAtBoundary);
         end_ = start_->CreateNextPageEndPosition(