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The content::ScopedWebCallbacks class is a helper for blink::WebCallbacks
and does not do anything strictly related to //content.
Moving it to //third_party/blink/public/platform makes it possible
to use it in all blink platform consumer, also the ones outside of //content,
such as //media/blink.
One example is //media/blink/webmediacapabilitiesclient_impl.cc which before
had to use specialized local copy of it.

This also adds two modifications to ScopedWebCallbacks:
* Make make_scoped_web_callbacks take a unique_ptr to WebCallbacks,
  instead of raw pointer that gets silently WrapUnique'ed inside,
  to make ownership passing more explicit,
* Modernize it to operate on OnceCallback, instead of Callback,
* Use default move constructor and move assignment.

Bug: 847211
Change-Id: I3d9b8a660dbfd66cf8721331777f36ea42c04bc8
Reviewed-on: https://chromium-review.googlesource.com/1104356
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568572}