Implement EXT_discard_framebuffer in D3D11 renderer

Change-Id: I52bcf0cfb1aa123e085a35730fdefb006b617c3c
Reviewed-on: https://chromium-review.googlesource.com/269232
Tested-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/renderer/FramebufferImpl.h b/src/libANGLE/renderer/FramebufferImpl.h
index 2d3c6e3..dddd9b8 100644
--- a/src/libANGLE/renderer/FramebufferImpl.h
+++ b/src/libANGLE/renderer/FramebufferImpl.h
@@ -39,6 +39,7 @@
     virtual void setDrawBuffers(size_t count, const GLenum *buffers) = 0;
     virtual void setReadBuffer(GLenum buffer) = 0;
 
+    virtual gl::Error discard(size_t count, const GLenum *attachments) = 0;
     virtual gl::Error invalidate(size_t count, const GLenum *attachments) = 0;
     virtual gl::Error invalidateSub(size_t count, const GLenum *attachments, const gl::Rectangle &area) = 0;