Instead of asserting that the app is calling glDeleteBuffers
on buffers we've seen created, ignore buffer names that we
haven't seen. This is allowed by the spec:

  Buffer objects are deleted by calling
    void DeleteBuffers(sizei n, const uint *buffers);
  Unused names in buffers are silently ignored, as is the value zero.

Test: SAKURA School Simulator MEC
Bug: b/188488065
Bug: angleproject:5987
Change-Id: Id4704e544f644dbe3edf6efdf70b5c3ef9ccaba7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2902442Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>