Remove DCHECKs from |PostLayout|

r789656 crrev.com/c/2302984 added a few DCHECKs to
|PostLayout| based on our existing assumptions, but they
did hit in DCHECK-enabled Canary.

While not all reasons are clear, we've got 20 crash logs
in a few days. This patch removes these DCHECKs.

Bug: 1107204
Change-Id: I78e247675278a5c9944bbc0d153a6d1543a8e033
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309494Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790297}

third_party/blink/renderer/core/layout/ng/ng_physical_fragment.cc

@@ -357,20 +357,23 @@ const NGPhysicalFragment* NGPhysicalFragment::PostLayout() const {
   const auto* layout_box = ToLayoutBoxOrNull(GetLayoutObject());
   if (UNLIKELY(!layout_box))
     return nullptr;
+  const wtf_size_t fragment_count = layout_box->PhysicalFragmentCount();
+  if (fragment_count == 0) {
+    // This should not happen, but DCHECK hits. crbug.com/1107204
+    return nullptr;
+  }
 
-  DCHECK_GT(layout_box->PhysicalFragmentCount(), 0u);
-  if (layout_box->PhysicalFragmentCount() == 1) {
+  if (fragment_count == 1) {
     const NGPhysicalFragment* post_layout = layout_box->GetPhysicalFragment(0);
     DCHECK(post_layout);
     if (UNLIKELY(post_layout && post_layout != this)) {
-      // Relayout boundary is the only case this can happen. crbug.com/829028
+      // This can happen at the relayout boundary crbug.com/829028
+      // but DCHECKing |IsRelayoutBoundary| hits. crbug.com/1107204
       DCHECK(layout_box->IsRelayoutBoundary());
       return post_layout;
     }
-  } else {
-    // TODO(crbug.com/829028): Block fragmentation not supported yet.
-    DCHECK(!layout_box->IsRelayoutBoundary());
   }
+  // TODO(crbug.com/829028): Block fragmentation not supported yet.
   DCHECK(std::any_of(layout_box->PhysicalFragments().begin(),
                      layout_box->PhysicalFragments().end(),
                      [this](const NGPhysicalFragment& fragment) {