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The MacViews "chrome" compile regressed in r301203

This adds a stub that just returns
scoped_ptr<ConstrainedWindowViewsClient>(nullptr) and fixes some
NativeWindow/NativeView inconsistencies (popup_manager->GetHostView()
returns a view).

The component is compiled on MacViews, since it will eventually be
needed. At link time for `chrome`, everything except the a pure virtual
destructor call will be culled since they are unused, but this approach
keeps component usage consistent with the test targets, which do have
the whole component.

note that components_unittests ConstrainedWindowViewsTest.* all pass on
a MacViews build except for one check against a dialog size
(constrained_window_views_unittest.cc line 142).

BUG=390755, 410499

Review URL: https://codereview.chromium.org/699553002

Cr-Commit-Position: refs/heads/master@{#302552}