ThinStrings: fix Factory::NewProperSubString

BUG=chromium:685504

Review-Url: https://codereview.chromium.org/2660823002
Cr-Commit-Position: refs/heads/master@{#42783}

src/factory.cc

@@ -795,6 +795,10 @@ Handle<String> Factory::NewProperSubString(Handle<String> str,
     str = Handle<String>(slice->parent(), isolate());
     offset += slice->offset();
   }
+  if (str->IsThinString()) {
+    Handle<ThinString> thin = Handle<ThinString>::cast(str);
+    str = handle(thin->actual(), isolate());
+  }
 
   DCHECK(str->IsSeqString() || str->IsExternalString());
   Handle<Map> map = str->IsOneByteRepresentation()

test/mjsunit/regress/regress-crbug-685504.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var v2 = 1073741823;
+var v13 = {};
+function f1(a, b) {
+  var v4 = a + b;
+  var v1 = v4.substring(20);
+  v2[v4];
+  return v1;
+}
+
+v5 = f1("abcdefghijklmnopqrstuvwxyz", "abcdefghijklmnopqrstuvwxyz");
+function f8(name, input, regexp) {
+  var v14 = input.match(regexp);
+  RegExp["$'"]}
+f8("CaptureGlobal", v5, v13, []["anama"]);