[builtins] Improve StringPrototypeEndsWith performance by adding a fastpath.

If the strings are both onebyte strings, then use memcmp on the underlying data stores.

BUG=chromium:657747

Review-Url: https://codereview.chromium.org/2502323002
Cr-Commit-Position: refs/heads/master@{#41033}

src/builtins/builtins-string.cc

@@ -770,8 +770,24 @@ BUILTIN(StringPrototypeEndsWith) {
   int start = end - search_string->length();
   if (start < 0) return isolate->heap()->false_value();
 
-  FlatStringReader str_reader(isolate, String::Flatten(str));
-  FlatStringReader search_reader(isolate, String::Flatten(search_string));
+  str = String::Flatten(str);
+  search_string = String::Flatten(search_string);
+
+  DisallowHeapAllocation no_gc;  // ensure vectors stay valid
+  String::FlatContent str_content = str->GetFlatContent();
+  String::FlatContent search_content = search_string->GetFlatContent();
+
+  if (str_content.IsOneByte() && search_content.IsOneByte()) {
+    Vector<const uint8_t> str_vector = str_content.ToOneByteVector();
+    Vector<const uint8_t> search_vector = search_content.ToOneByteVector();
+
+    return isolate->heap()->ToBoolean(memcmp(str_vector.start() + start,
+                                             search_vector.start(),
+                                             search_string->length()) == 0);
+  }
+
+  FlatStringReader str_reader(isolate, str);
+  FlatStringReader search_reader(isolate, search_string);
 
   for (int i = 0; i < search_string->length(); i++) {
     if (str_reader.Get(start + i) != search_reader.Get(i)) {