Vulkan: Encapsulate ShareGroup::mContexts Instead of return mContexts, this add addContext/removeContext API and changed getContexts to const function. No functional change expected, but clean up only. Bug: b/223428306 Change-Id: I020c4bc781f58352089980c9d8ff22bcb183dbe6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3590970 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>