Revert "[es6] Optimize String{Starts, Ends}With"

This reverts commit b7db5cd9
(https://codereview.chromium.org/1324353002/).

Our internal dashboard shows that this patch has introduced massive (3x) performance regressions for string ops. This is probably due to it repeatedly invoking %_StringCharCodeAt in a loop, which is far from cheap (has to dispatch on one of our 30+ string representations each time).

TBR=dehrenberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1439083003

Cr-Commit-Position: refs/heads/master@{#31981}

src/js/string.js

@@ -17,6 +17,8 @@ var InternalArray = utils.InternalArray;
 var InternalPackedArray = utils.InternalPackedArray;
 var MakeRangeError;
 var MakeTypeError;
+var MathMax;
+var MathMin;
 var RegExpExec;
 var RegExpExecNoTests;
 var RegExpLastMatchInfo;
@@ -27,6 +29,8 @@ utils.Import(function(from) {
   ArrayJoin = from.ArrayJoin;
   MakeRangeError = from.MakeRangeError;
   MakeTypeError = from.MakeTypeError;
+  MathMax = from.MathMax;
+  MathMin = from.MathMin;
   RegExpExec = from.RegExpExec;
   RegExpExecNoTests = from.RegExpExecNoTests;
   RegExpLastMatchInfo = from.RegExpLastMatchInfo;
@@ -879,21 +883,13 @@ function StringStartsWith(searchString /* position */) {  // length == 1
   }
 
   var s_len = s.length;
-  if (pos < 0) pos = 0;
-  if (pos > s_len) pos = s_len;
+  var start = MathMin(MathMax(pos, 0), s_len);
   var ss_len = ss.length;
-
-  if (ss_len + pos > s_len) {
+  if (ss_len + start > s_len) {
     return false;
   }
 
-  for (var i = 0; i < ss_len; i++) {
-    if (%_StringCharCodeAt(s, pos + i) !== %_StringCharCodeAt(ss, i)) {
-      return false;
-    }
-  }
-
-  return true;
+  return %_SubString(s, start, start + ss_len) === ss;
 }
 
 
@@ -917,22 +913,14 @@ function StringEndsWith(searchString /* position */) {  // length == 1
     }
   }
 
-  if (pos < 0) pos = 0;
-  if (pos > s_len) pos = s_len;
+  var end = MathMin(MathMax(pos, 0), s_len);
   var ss_len = ss.length;
-  pos = pos - ss_len;
-
-  if (pos < 0) {
-    return false;
-  }
-
-  for (var i = 0; i < ss_len; i++) {
-    if (%_StringCharCodeAt(s, pos + i) !== %_StringCharCodeAt(ss, i)) {
+  var start = end - ss_len;
+  if (start < 0) {
     return false;
   }
-  }
 
-  return true;
+  return %_SubString(s, start, start + ss_len) === ss;
 }