Make ElementInnerTextCollector::IsAfterWhiteSpace() to use IsInlineTextBox()

This patch changes  |ElementInnerTextCollector::IsAfterWhiteSpace()| to use
|IsInlineTextBox()| instead of |InlineBox::IsText()| to detect |InlineTextBox|
type properly.

Note: |InlineBox::IsText()| is used for handling <br> in quirk mode, and list
marker.

This patch is follow-up of the patch[1] to fix the bug found by Cluster Fuzz.

[1] http://crrev.com/c/1114673 Implement Element#innerText to conform the spec

Bug: 877498
Change-Id: I484a93afba351b74a8b3cae0c6736e3ccdfde45d
Reviewed-on: https://chromium-review.googlesource.com/1189242Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586222}

third_party/blink/renderer/core/editing/BUILD.gn

@@ -357,6 +357,7 @@ jumbo_source_set("unit_tests") {
     "editing_style_test.cc",
     "editing_utilities_test.cc",
     "editor_test.cc",
+    "element_inner_text_test.cc",
     "ephemeral_range_test.cc",
     "frame_caret_test.cc",
     "frame_selection_test.cc",

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

@@ -210,7 +210,7 @@ bool ElementInnerTextCollector::IsAfterWhiteSpace(
   }
   if (InlineTextBox* text_box = layout_text.FirstTextBox()) {
     const InlineBox* previous = text_box->PrevLeafChild();
-    if (!previous || !previous->IsText())
+    if (!previous || !previous->IsInlineTextBox())
       return false;
     return EndsWithWhiteSpace(ToInlineTextBox(*previous));
   }

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

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "third_party/blink/renderer/core/dom/element.h"
+
+#include "third_party/blink/renderer/core/editing/testing/editing_test_base.h"
+
+namespace blink {
+
+class ElementInnerTest : public EditingTestBase {};
+
+// http://crbug.com/877498
+TEST_F(ElementInnerTest, ListItemWithLeadingWhiteSpace) {
+  SetBodyContent("<li id=target> abc</li>");
+  Element& target = *GetDocument().getElementById("target");
+  EXPECT_EQ("abc", target.innerText());
+}
+
+}  // namespace blink