Don't use AddSubstring for external natives

The input string is not necessarily zero terminated

BUG=chromium:443230
R=adamk@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/795383003

Cr-Commit-Position: refs/heads/master@{#25879}

src/natives-external.cc

@@ -82,15 +82,14 @@ class NativesStore {
   NativesStore() : debugger_count_(0) {}
 
   Vector<const char> NameFromId(const byte* id, int id_length) {
-    Vector<char> name(Vector<char>::New(id_length + 11));
-    SimpleStringBuilder builder(name.start(), name.length());
-    builder.AddString("native ");
-    builder.AddSubstring(reinterpret_cast<const char*>(id), id_length);
-    builder.AddString(".js");
-    builder.Finalize();
-    // SimpleStringBuilder wants zero-byte; the caller does not.
-    DCHECK(name[name.length() - 1] == '\0');
-    name.Truncate(name.length() - 1);
+    const char native[] = "native ";
+    const char extension[] = ".js";
+    Vector<char> name(Vector<char>::New(id_length + sizeof(native) - 1 +
+                                        sizeof(extension) - 1));
+    memcpy(name.start(), native, sizeof(native) - 1);
+    memcpy(name.start() + sizeof(native) - 1, id, id_length);
+    memcpy(name.start() + sizeof(native) - 1 + id_length, extension,
+           sizeof(extension) - 1);
     return Vector<const char>::cast(name);
   }
 

src/utils.h

@@ -863,7 +863,7 @@ class SimpleStringBuilder {
   // compute the length of the input string.
   void AddString(const char* s);
 
-  // Add the first 'n' characters of the given string 's' to the
+  // Add the first 'n' characters of the given 0-terminated string 's' to the
   // builder. The input string must have enough characters.
   void AddSubstring(const char* s, int n);