Add dummy attachment in FramebufferD3D as a private member and release
the dummy attchment in destory of FramebufferD3D.

BUG=angleproject:2282

TEST=FramebufferTest_ES31.RenderingLimitToDefaultFBOSizeWithNoAttachments/ES3_1_D3D11
TEST=dEQP-GLES31.functional.fbo.no_attachments.*

Change-Id: I3a17282ef132185fbc25f4076f624e53661d4b20
Reviewed-on: https://chromium-review.googlesource.com/831847
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>