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This change adds two new methods to WidgetDelegate, both of which allow
setting the WidgetDelegate's contents view, thus removing the need to
override GetContentsView(). The two new methods are:

* SetContentsView(unique_ptr<View>), which takes ownership of the given
  View
* SetContentsView(View*), which doesn't

Each method can only be used with the appropriate ownership protocol, so
SetContentsView(unique_ptr<View>) will DCHECK if the provided view is
owned_by_client(), and vice versa for SetContentsView(View*). This
effectively forces clients into one of these two ownership protocols and
away from the "sometimes owning" pointer semantics of overriding
GetContentsView().

This is part 2 of the following plan:
1) Introduce WidgetDelegate::TransferOwnershipOfContentsView() and
   migrate to it in places that are trying to take ownership
2) Introduce WidgetDelegate::SetContentsView(), which will either:
   a. Store the provided view in a unique_ptr, if it is not owned
      by client, or
   b. Store the provided view in a raw pointer, if it is owned by
      client
3) Replace all existing overrides of GetContentsView() with uses of
   SetContentsView()
4) Require that !owned_by_client() in SetContentsView(), which will
   likely require intense surgery of client classes
5) Have SetContentsView() take and TransferOwnershipOfContentsView()
   return a unique_ptr<View> rather than a View*
as laid out in 1206dbae.

Bug: 1075649
Change-Id: Ifa35af1d87ad958a7d544ed74d6b1f980e416463
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2449919
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814274}