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

Source/platform/text/SegmentedStringTest.cpp

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 29 matching lines @@
     SegmentedString original(String("cde"));
     {
         SegmentedString copied(original);
         SegmentedString assigned;
         assigned = original;
         EXPECT_EQ("cde", original.toString());
         EXPECT_EQ('c', original.currentChar());
         EXPECT_EQ('c', copied.currentChar());
         EXPECT_EQ('c', assigned.currentChar());
     }
+    original.push('b');
+    {
+        SegmentedString copied(original);
+        SegmentedString assigned;
+        assigned = original;
+        EXPECT_EQ("bcde", original.toString());
+        EXPECT_EQ('b', original.currentChar());
+        EXPECT_EQ('b', copied.currentChar());
+        EXPECT_EQ('b', assigned.currentChar());
+    }
     original.push('a');
     {
         SegmentedString copied(original);
         SegmentedString assigned;
         assigned = original;
-        EXPECT_EQ("acde", original.toString());
-        EXPECT_EQ('a', original.currentChar());
-        EXPECT_EQ('a', copied.currentChar());
-        EXPECT_EQ('a', assigned.currentChar());
-    }
-    original.push('b');
-    {
-        // Two consecutive push means to push the second char *after* the
-        // first char in SegmentedString.
-        SegmentedString copied(original);
-        SegmentedString assigned;
-        assigned = original;
         EXPECT_EQ("abcde", original.toString());
         EXPECT_EQ('a', original.currentChar());
         EXPECT_EQ('a', copied.currentChar());
         EXPECT_EQ('a', assigned.currentChar());
     }
     original.advance();
     {
         SegmentedString copied(original);
         SegmentedString assigned;
         assigned = original;
         EXPECT_EQ("bcde", original.toString());
         EXPECT_EQ('b', original.currentChar());
         EXPECT_EQ('b', copied.currentChar());
         EXPECT_EQ('b', assigned.currentChar());
     }
 }
 
 } // namespace blink