[es6] Use SubString in String{Starts,Ends}With

Much faster and constant than always searching the whole string

````
var allCodePoints = [];
for (var i = 0; i < 65536; i++) allCodePoints[i] = i;
var allCharsString = String.fromCharCode.apply(String, allCodePoints);

function bench(search) {
  var counter = 0;
  print(search + " found at " + allCharsString.startsWith(search));
  var start = Date.now();
  while (counter++ < 5000000) {
    allCharsString.startsWith(search);
  }
  var end = Date.now();
  print(end - start);
  return counter;
}

print("single character");
bench("\u0000");
bench("\u0050");
bench("\u1000");
````

OLD

single character
 found at true
374
P found at false
559
က found at false
13492

NEW

single character
 found at true
261
P found at false
146
က found at false
146

BUG=v8:4384
LOG=N

Review URL: https://codereview.chromium.org/1321853006

Cr-Commit-Position: refs/heads/master@{#30599}

src/string.js

@@ -997,7 +997,7 @@ function StringStartsWith(searchString /* position */) {  // length == 1
     return false;
   }
 
-  return %StringIndexOf(s, ss, start) === start;
+  return %_SubString(s, start, start + ss_len) === ss;
 }
 
 
@@ -1028,7 +1028,7 @@ function StringEndsWith(searchString /* position */) {  // length == 1
     return false;
   }
 
-  return %StringLastIndexOf(s, ss, start) === start;
+  return %_SubString(s, start, start + ss_len) === ss;
 }