Vulkan: Fix support for framebuffer discard extension

Bug: angleproject:3664
Change-Id: Ifc08a53771c1e114c4169780e7fc984f2552b263
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1688182
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

src/libANGLE/renderer/vulkan/FramebufferVk.cpp

@@ -182,8 +182,7 @@ angle::Result FramebufferVk::discard(const gl::Context *context,
                                      size_t count,
                                      const GLenum *attachments)
 {
-    ANGLE_VK_UNREACHABLE(vk::GetImpl(context));
-    return angle::Result::Stop;
+    return invalidate(context, count, attachments);
 }
 
 angle::Result FramebufferVk::invalidate(const gl::Context *context,