Move native window check to the DisplayImpl.

This hides the D3D-specific methods from the EGL side of things.

BUG=angle:773

Change-Id: I5a1c2bbff865e02bc1a07b3295347469ef9792e3
Reviewed-on: https://chromium-review.googlesource.com/232943
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Remove polymorphism of egl::Display.

This was causing a warning on GCC, and is no longer necessary.

BUG=angle:773

Change-Id: I6cb09e79bff9035e3e38966dcad2cc7ba74701a1
Reviewed-on: https://chromium-review.googlesource.com/232944
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>