Fix update external string call in morph string test.

Bug: chromium:845409
Change-Id: Ic9b1fa0f669c8b9a82ed91d3f348dd070fc9267f
Reviewed-on: https://chromium-review.googlesource.com/1135247
Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54419}

test/cctest/test-api.cc

@@ -15654,9 +15654,7 @@ static void MorphAString(i::String* string,
     string->set_map(roots.external_string_map());
     i::ExternalTwoByteString* morphed =
          i::ExternalTwoByteString::cast(string);
-    CcTest::heap()->UpdateExternalString(morphed, morphed->resource()->length(),
-                                         0);
-
+    CcTest::heap()->UpdateExternalString(morphed, string->length(), 0);
     morphed->SetResource(uc16_resource);
   } else {
     // Check old map is not internalized or long.
@@ -15664,8 +15662,7 @@ static void MorphAString(i::String* string,
     // Morph external string to be one-byte string.
     string->set_map(roots.external_one_byte_string_map());
     i::ExternalOneByteString* morphed = i::ExternalOneByteString::cast(string);
-    CcTest::heap()->UpdateExternalString(morphed,
-                                         morphed->resource()->length() * 2, 0);
+    CcTest::heap()->UpdateExternalString(morphed, string->length(), 0);
     morphed->SetResource(one_byte_resource);
   }
 }