[reland] SegmentedString::push() should always push a char in front

Source/core/html/parser/HTMLEntityParserTest.cpp

+// Copyright 2015 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 "config.h"
+#include "core/html/parser/HTMLEntityParser.h"
+
+#include <gtest/gtest.h>
+
+namespace blink {
+
+TEST(HTMLEntityParserTest, ConsumeHTMLEntityIncomplete)
+{
+    String original("am"); // Incomplete by purpose.
+    SegmentedString src(original);
+
+    DecodedHTMLEntity entity;
+    bool notEnoughCharacters = false;
+    bool success = consumeHTMLEntity(src, entity, notEnoughCharacters);
+    EXPECT_TRUE(notEnoughCharacters);
+    EXPECT_FALSE(success);
+
+    // consumeHTMLEntity should recover the original SegmentedString state if failed.
+    EXPECT_EQ(original, src.toString());
+}
+
+} // namespace blink