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

Source/platform/text/SegmentedString.h

 /*
     Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
 
     This library is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 172 matching lines @@
     void close();
 
     void append(const SegmentedString&);
     void prepend(const SegmentedString&);
 
     bool excludeLineNumbers() const { return m_currentString.excludeLineNumbers(); }
     void setExcludeLineNumbers();
 
     void push(UChar c)
     {
+        ASSERT(c);
+
         if (!m_pushedChar1) {
-            m_pushedChar1 = c;
-            m_currentChar = m_pushedChar1 ? m_pushedChar1 : m_currentString.getCurrentChar();
+            m_currentChar = m_pushedChar1 = c;
             updateSlowCaseFunctionPointers();
         } else {
             ASSERT(!m_pushedChar2);
-            m_pushedChar2 = c;
+            m_pushedChar2 = m_pushedChar1;
+            m_currentChar = m_pushedChar1 = c;
         }
     }
 
     bool isEmpty() const { return m_empty; }
     unsigned length() const;
 
     bool isClosed() const { return m_closed; }
 
     enum LookAheadResult {
         DidNotMatch,
@@ skipping 198 matching lines @@
     bool m_closed;
     bool m_empty;
     unsigned char m_fastPathFlags;
     void (SegmentedString::*m_advanceFunc)();
     void (SegmentedString::*m_advanceAndUpdateLineNumberFunc)();
 };
 
 }
 
 #endif