Add EditingTestBase::GetCaretTextFromBody

This patch adds the above function, so that when writing unit tests, we
can check the correctness of functions returning positions in an easier
way.

This is a preparation patch for crrev.com/c/791856

Bug: 
Change-Id: If7de011090dca69830a4d8a404f1321934971488
Reviewed-on: https://chromium-review.googlesource.com/706575Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519579}

third_party/WebKit/Source/core/editing/BUILD.gn

@@ -391,6 +391,7 @@ jumbo_source_set("unit_tests") {
     "suggestion/TextSuggestionControllerTest.cpp",
     "testing/EditingTestBase.cpp",
     "testing/EditingTestBase.h",
+    "testing/EditingTestBaseTest.cpp",
     "testing/SelectionSample.cpp",
     "testing/SelectionSample.h",
     "testing/SelectionSampleTest.cpp",

third_party/WebKit/Source/core/editing/testing/EditingTestBase.cpp

@@ -77,6 +77,14 @@ std::string EditingTestBase::GetSelectionTextInFlatTreeFromBody(
                                                      selection);
 }
 
+std::string EditingTestBase::GetCaretTextFromBody(
+    const Position& position) const {
+  DCHECK(position.IsValidFor(GetDocument()))
+      << "A valid position must be provided " << position;
+  return GetSelectionTextFromBody(
+      SelectionInDOMTree::Builder().Collapse(position).Build());
+}
+
 ShadowRoot* EditingTestBase::CreateShadowRootForElementWithIDAndSetInnerHTML(
     TreeScope& scope,
     const char* host_element_id,

third_party/WebKit/Source/core/editing/testing/EditingTestBase.h

@@ -51,6 +51,9 @@ class EditingTestBase : public PageTestBase {
   SelectionInDOMTree SetSelectionText(HTMLElement*,
                                       const std::string& selection_text);
 
+  // Returns selection text for child nodes of BODY with specific |Position|.
+  std::string GetCaretTextFromBody(const Position&) const;
+
   // Returns selection text for child nodes of BODY with specified
   // |SelectionInDOMTree|.
   std::string GetSelectionTextFromBody(const SelectionInDOMTree&) const;

third_party/WebKit/Source/core/editing/testing/EditingTestBaseTest.cpp

+// Copyright 2017 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 "core/editing/testing/EditingTestBase.h"
+
+#include "core/editing/Position.h"
+
+namespace blink {
+
+class EditingTestBaseTest : public EditingTestBase {};
+
+TEST_F(EditingTestBaseTest, GetCaretTextFromBody) {
+  SetBodyContent("<div>foo</div>");
+  Element* const div = GetDocument().QuerySelector("div");
+  Node* const foo = div->firstChild();
+  EXPECT_EQ("|<div>foo</div>",
+            GetCaretTextFromBody(Position::BeforeNode(*div)));
+
+  // TODO(editing-dev): Consider different serialization for the following two
+  // positions.
+  EXPECT_EQ("<div>|foo</div>",
+            GetCaretTextFromBody(Position::FirstPositionInNode(*div)));
+  EXPECT_EQ("<div>|foo</div>", GetCaretTextFromBody(Position(foo, 0)));
+
+  // TODO(editing-dev): Consider different serialization for the following two
+  // positions.
+  EXPECT_EQ("<div>foo|</div>", GetCaretTextFromBody(Position(foo, 3)));
+  EXPECT_EQ("<div>foo|</div>",
+            GetCaretTextFromBody(Position::LastPositionInNode(*div)));
+
+  EXPECT_EQ("<div>foo</div>|", GetCaretTextFromBody(Position::AfterNode(*div)));
+}
+
+// TODO(editing-dev): Add demos of other functions of EditingTestBase.
+
+}  // namespace blink