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This is done in a comparable way to NativeViewHostAura, but using a
ReparentNSView helper function which reparents an NSView to a
NativeWidgetMac's BridgedContentView (or to `nil`).

Earlier attempts tried to use NativeWidgetPrivate::ReparentNativeView()
which turns out to be a bad idea, since it's really designed to reparent
windows (aura::Windows or HWNDs). Mac's child window support is
insufficient for what toolkit-views expects, and WebContents on Mac are
managed by an NSView in any case, not an NSWindow.

Along with logic currently in render_widget_host_view_mac.mm and
web_contents_view_mac.mm, reparenting renderer views needs to be done in
the native view hierarchy, not the window hierarchy.

This CL no longer tries to enable the current
native_view_host_unittest.cc (tests there assume ReparentNativeView is
used), but adds native_view_host_mac_unittest.mm to give test coverage,
and to inspect properties at the Cocoa layer. Common parts of the 3
_unittest.* files are refactored into a NativeViewHostTestBase class.

Some other views_unittests are also made passing by implementing a
NativeViewHost::CreateWrapper that doesn't just return NULL:
4xFocusTraverslTest and NativeWidgetTest.GetTopLevelNativeWidget2

Before: 455 tests run 22 tests failed 21 tests crashed. After: 458 tests
run 22 tests failed 16 tests crashed.

BUG=378134, 415024

Review URL: https://codereview.chromium.org/530933002

Cr-Commit-Position: refs/heads/master@{#295618}