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/Framebuffer.cpp b/src/libANGLE/Framebuffer.cpp
index ab717e1..635c8c6 100644
--- a/src/libANGLE/Framebuffer.cpp
+++ b/src/libANGLE/Framebuffer.cpp
@@ -497,6 +497,11 @@
     return mImpl->checkStatus();
 }
 
+Error Framebuffer::discard(size_t count, const GLenum *attachments)
+{
+    return mImpl->discard(count, attachments);
+}
+
 Error Framebuffer::invalidate(size_t count, const GLenum *attachments)
 {
     return mImpl->invalidate(count, attachments);