[LayoutNG] Fix PreviousOnLineInlineNG

It returned nullptr for any objects laid out by LayoutNG.

`AXPosition::AtStartOfLine()` relies on this, and that it
fails to return the correct value in LayoutNG.

This patch fixes following content_browsertests with LayoutNG:
  AccessibilityAuraLinuxBrowserTest.TestParagraphTextAtOffsetWithBoundaryLine
  AccessibilityWinBrowserTest.TestParagraphTextAtOffsetWithBoundaryLine

Change-Id: I409682232a29f84cda7e2d547ffd6a264357b3d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1608867Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659579}

third_party/blink/renderer/modules/accessibility/ax_layout_object.cc

@@ -1318,7 +1318,7 @@ static AXObject* PreviousOnLineInlineNG(const AXObject& ax_object) {
   DCHECK(!layout_object.IsListMarkerIncludingNG()) << layout_object;
   DCHECK(ShouldUseLayoutNG(layout_object)) << layout_object;
   const auto fragments = NGPaintFragment::InlineFragmentsFor(&layout_object);
-  if (fragments.IsEmpty() || fragments.IsInLayoutNGInlineFormattingContext())
+  if (fragments.IsEmpty() || !fragments.IsInLayoutNGInlineFormattingContext())
     return nullptr;
   for (NGPaintFragmentTraversalContext runner =
            NGPaintFragmentTraversal::PreviousInlineLeafOf(