Make TextOffsetMapping skip anonymous block flow wrappers for pseudo elements

In some cases, we create anonymous block flow wrappers for float pseudo
elements. Such wrappers should be skipped by TextOffsetMapping.

Bug: 900906
Change-Id: I7f9ce5906ed8b2daaff3b0c9310e639d932ae5d0
Reviewed-on: https://chromium-review.googlesource.com/c/1315701
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605262}

third_party/blink/renderer/core/editing/text_offset_mapping.cc

@@ -103,9 +103,10 @@ const LayoutBlockFlow* ComputeInlineContentsAsBlockFlow(
       block_flow.IsFloatingOrOutOfFlowPositioned()) {
     const LayoutBlockFlow& root_block_flow =
         RootInlineContentsContainerOf(block_flow);
-    DCHECK(CanBeInlineContentsContainer(root_block_flow))
-        << layout_object << " block_flow=" << block_flow
-        << " root_block_flow=" << root_block_flow;
+    // Skip |root_block_flow| if it's an anonymous wrapper created for
+    // pseudo elements. See test AnonymousBlockFlowWrapperForFloatPseudo.
+    if (!CanBeInlineContentsContainer(root_block_flow))
+      return nullptr;
     return &root_block_flow;
   }
   if (!CanBeInlineContentsContainer(block_flow))

third_party/blink/renderer/core/editing/text_offset_mapping_test.cc

@@ -230,6 +230,20 @@ TEST_P(ParameterizedTextOffsetMappingTest, RangeOfEmptyBlock) {
             TextOffsetMapping::FindBackwardInlineContents(position));
 }
 
+// http://crbug.com/900906
+TEST_P(ParameterizedTextOffsetMappingTest,
+       AnonymousBlockFlowWrapperForFloatPseudo) {
+  InsertStyleElement("table::after{content:close-quote;float:right;}");
+  const PositionInFlatTree position =
+      ToPositionInFlatTree(SetCaretTextToBody("<table></table>|foo"));
+  const TextOffsetMapping::InlineContents inline_contents =
+      TextOffsetMapping::FindBackwardInlineContents(position);
+  ASSERT_TRUE(inline_contents.IsNotNull());
+  const TextOffsetMapping::InlineContents previous_contents =
+      TextOffsetMapping::InlineContents::PreviousOf(inline_contents);
+  EXPECT_TRUE(previous_contents.IsNull());
+}
+
 // http://crbug.com/832497
 TEST_P(ParameterizedTextOffsetMappingTest, RangeWithCollapsedWhitespace) {
   // Whitespaces after <div> is collapsed.