[editing] Add test for CHECK failure in CreateMarkup

The change from bug 1002937 made this code trigger a CHECK failure in
CreateMarkupAlgorithm::CreateMarkup. It's fine now since it had to be
reverted because of bug 1161370, but adding test coverage will be useful
to avoid the problem when relanding the patch.

Bug: 1155665
Change-Id: I6f39eb7c29bc607fa41d662fedd48c63611d8377
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632613Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Cr-Commit-Position: refs/heads/master@{#844505}

third_party/blink/renderer/core/editing/commands/composite_edit_command_test.cc

@@ -144,7 +144,7 @@ TEST_F(CompositeEditCommandTest,
 }
 
 TEST_F(CompositeEditCommandTest,
-       MoveParagraphContentsToNewBlockWithUAShadowDOM) {
+       MoveParagraphContentsToNewBlockWithUAShadowDOM1) {
   SetBodyContent("<object contenteditable><input></object>");
   base::RunLoop().RunUntilIdle();
 
@@ -160,6 +160,23 @@ TEST_F(CompositeEditCommandTest,
             GetDocument().body()->innerHTML());
 }
 
+TEST_F(CompositeEditCommandTest,
+       MoveParagraphContentsToNewBlockWithUAShadowDOM2) {
+  GetDocument().setDesignMode("on");
+  SetBodyContent("<span></span><button><meter></meter></button>");
+
+  SampleCommand& sample = *MakeGarbageCollected<SampleCommand>(GetDocument());
+  Element* button = GetDocument().QuerySelector("button");
+  Position pos = Position(button, 0);
+  EditingState editing_state;
+
+  // Should not crash
+  sample.MoveParagraphContentsToNewBlockIfNecessary(pos, &editing_state);
+  EXPECT_FALSE(editing_state.IsAborted());
+  EXPECT_EQ("<div></div><span></span><button><meter></meter></button>",
+            GetDocument().body()->innerHTML());
+}
+
 TEST_F(CompositeEditCommandTest, InsertNodeOnDisconnectedParent) {
   SetBodyContent("<p><b></b></p>");
   SampleCommand& sample = *MakeGarbageCollected<SampleCommand>(GetDocument());