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

Source/platform/text/SegmentedString.h

Index: Source/platform/text/SegmentedString.h
diff --git a/Source/platform/text/SegmentedString.h b/Source/platform/text/SegmentedString.h
index 88ffb605caae433f8f3e524b58b758c778e92196..7daa101f2a3f980cd9daee06b778a56e77549102 100644
--- a/Source/platform/text/SegmentedString.h
+++ b/Source/platform/text/SegmentedString.h
@@ -190,13 +190,15 @@ public:
 
     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;
         }
     }