Purge unused code once associated with NativeWidgetWin.

src/ui/views/native_widget_win.h doesn't exist any more (since r247647).
This removes some remaining code that was only used by NativeWidgetWin,
and purges remaining references to NativeWidgetWin.

Removes views::DropTargetWin and unused unit tests.

BUG=330735

Review URL: https://codereview.chromium.org/185463009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255020 0039d316-1c4b-4281-b951-d872f2087c98
Settings Override API: limit it to Windows.

BUG=267510

Review URL: https://codereview.chromium.org/186613004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255021 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255021