- Metal's occlusion(called visibility) query operates per render pass.
  Implementation details are in
  src/libANGLE/renderer/metal/doc/OcclusionQueries.md.
- New tests:
  - OcclusionQueriesTest.ClearNotCounted.
  - OcclusionQueriesTest.MultiQueries (failure on OpenGL/D3D11
    back-end).

Bug: angleproject:2634
Change-Id: Idd1327b5472d0e8c2b69307a7f04a1da4a847a40
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2336121
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
Reviewed-by: back sept 10 - Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>