Make FramebufferAttachmentObject not refcountable

Instead the refcount is done via callbacks. This allows Surface to
ignore this refcounting which will be useful in a follow-up CL.

BUG=angleproject:891

Change-Id: I39b028476e0e3ab1837c033e1121ea21e54d7970
Reviewed-on: https://chromium-review.googlesource.com/291651
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/Image.cpp b/src/libANGLE/Image.cpp
index 471bc76..a9448e3 100644
--- a/src/libANGLE/Image.cpp
+++ b/src/libANGLE/Image.cpp
@@ -17,8 +17,7 @@
 
 namespace egl
 {
-ImageSibling::ImageSibling(GLuint id)
-    : gl::FramebufferAttachmentObject(id), mSourcesOf(), mTargetOf()
+ImageSibling::ImageSibling(GLuint id) : RefCountObject(id), mSourcesOf(), mTargetOf()
 {
 }