[LayoutNG] Simplify NGInlineFragmentTraversal

Old implementation of NGInlineFragmentTraversal traverses all descendants
of an NGPhysicalFragment, and uses a filter to decide which fragments to
emit. As a result, the implementation is convoluted.

This patch rewrites it by using a visitor that can decide the traversal
process, and also when/what to emit. As a result, the implementation
is greatly simplified.

Bug: 851075
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I67a1331f906981e48cc366697234f8d5996c02ea
Reviewed-on: https://chromium-review.googlesource.com/1097877
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568207}

third_party/blink/renderer/core/layout/ng/inline/ng_inline_fragment_traversal.h

@@ -44,7 +44,7 @@ class CORE_EXPORT NGInlineFragmentTraversal {
 
   // Returns list of inline fragments produced from the specified LayoutObject.
   // The search is restricted in the subtree of |container|.
-  static Vector<NGPhysicalFragmentWithOffset, 1> SelfFragmentsOf(
+  static Vector<NGPhysicalFragmentWithOffset> SelfFragmentsOf(
       const NGPhysicalContainerFragment& container,
       const LayoutObject* target);
 };