This allows iterating through the entries without creating a copy
of the vector. This was causing an assertion in FrameCapture when
deconstructing the copy, which we don't need.

  RefCountObject.h (~BindingPointer): ! Assert failed in ~BindingPointer
  (src/libANGLE/RefCountObject.h:93): mObject == nullptr

It also probably matches the intent of the function.

Test: Aztec Ruins MEC
Bug: angleproject:4048
Change-Id: I9588af3859b4d3edb5859530def1bb3992530403
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2622610Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>