The previous NativeWindow class included D3D11 headers while being included in
all D3D backds and had platform-dependent includes and members.  This
turns it into an abstract class that only implements the minimal
functionality for each renderer.

BUG=angleproject:1345

Change-Id: I8f20339dd6bba719e574a1dcb3ec859897c9228f
Reviewed-on: https://chromium-review.googlesource.com/336780Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>