[parser] Proper bounds check and casts for stream creation.

If a negative value is passed as end position it may get past the end
without triggering any DCHECK due to int to size_t cast.

BUG=v8:6093

Change-Id: I0c6be0e8442049cc4b7fc87593ad018bce4b677e
Reviewed-on: https://chromium-review.googlesource.com/494108
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45068}

src/parsing/scanner-character-streams.cc

@@ -816,16 +816,20 @@ Utf16CharacterStream* ScannerStream::For(Handle<String> data) {
 Utf16CharacterStream* ScannerStream::For(Handle<String> data, int start_pos,
                                          int end_pos) {
   DCHECK(start_pos >= 0);
+  DCHECK(start_pos <= end_pos);
   DCHECK(end_pos <= data->length());
   if (data->IsExternalOneByteString()) {
     return new ExternalOneByteStringUtf16CharacterStream(
-        Handle<ExternalOneByteString>::cast(data), start_pos, end_pos);
+        Handle<ExternalOneByteString>::cast(data),
+        static_cast<size_t>(start_pos), static_cast<size_t>(end_pos));
   } else if (data->IsExternalTwoByteString()) {
     return new ExternalTwoByteStringUtf16CharacterStream(
-        Handle<ExternalTwoByteString>::cast(data), start_pos, end_pos);
+        Handle<ExternalTwoByteString>::cast(data),
+        static_cast<size_t>(start_pos), static_cast<size_t>(end_pos));
   } else {
     // TODO(vogelheim): Maybe call data.Flatten() first?
-    return new GenericStringUtf16CharacterStream(data, start_pos, end_pos);
+    return new GenericStringUtf16CharacterStream(
+        data, static_cast<size_t>(start_pos), static_cast<size_t>(end_pos));
   }
 }