This allows applications to specify that they want to render to the screen
using DirectComposition. This can save power by allowing the use of
DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL and DirectFlip, but has other side effects -
windows must be owned by the current process, and contents drawn by GDI are
shown underneatch the window.

BUG=angleproject:1178
Change-Id: I2e45d92c9f92f27eedaf50f0a866c20c43f3aab5
Reviewed-on: https://chromium-review.googlesource.com/317651Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: John Bauman <jbauman@chromium.org>