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Major changes in this CL are:
- Drops [CustomConstructor] from MutationObserver.idl.
- Replaces hand-written V8MutationCallback with code-generated
  MutationCallback. Note that the new (code-generated)
  MutationCallback is different from the old (hand-written)
  MutationCallback.
- Rewrote the old (hand-written) MutationCallback as
  MutationObserver::Delegate.  Note that the name
  "MutationCallback" is taken by the new (code-generated)
  MutationCallback.

This CL mostly follows the same way as https://crrev.com/c/581128 .

Bug: 
Change-Id: I3f8b36044499c50baade331e2bfafbbed4271e0d
Reviewed-on: https://chromium-review.googlesource.com/590832Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491297}