- Provide MapData::SerializeDescriptors method for serializing the whole
  descriptor array.
- Trigger this in JSObjectData::SerializeAsBoilerplate.
- Further make things more consistent across the broker.

Bug: v8:7790
Change-Id: Ie6499da8857f7c6561f7c44922aeffcea4876be7
Reviewed-on: https://chromium-review.googlesource.com/1199102
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55756}