SegmentedString::prepend should know if the prepended string is new or previously consumed

third_party/WebKit/Source/core/html/parser/BackgroundHTMLInputStream.cpp

Index: third_party/WebKit/Source/core/html/parser/BackgroundHTMLInputStream.cpp
diff --git a/third_party/WebKit/Source/core/html/parser/BackgroundHTMLInputStream.cpp b/third_party/WebKit/Source/core/html/parser/BackgroundHTMLInputStream.cpp
index 56f10af2f23b3fb30eb65d5d249676b23803bc73..971a71dadc1b4438b7f3da3d34484c9fbc584564 100644
--- a/third_party/WebKit/Source/core/html/parser/BackgroundHTMLInputStream.cpp
+++ b/third_party/WebKit/Source/core/html/parser/BackgroundHTMLInputStream.cpp
@@ -90,8 +90,10 @@ void BackgroundHTMLInputStream::rewindTo(HTMLInputCheckpoint checkpointIndex, co
         m_current.append(SegmentedString(m_segments[i]));
     }
 
-    if (!unparsedInput.isEmpty())
-        m_current.prepend(SegmentedString(unparsedInput));
+    if (!unparsedInput.isEmpty()) {
+        m_current.prepend(SegmentedString(unparsedInput),
+            SegmentedString::PrependType::NewInput);
+    }
 
     if (isClosed && !m_current.isClosed())
         m_current.close();