Create Context::getActiveLinkedProgram() In preparation for support Program Pipeline Objects, the new function Context::getActiveLinkedProgram() is being created that will handle returning the correct Program whether there is currently a Program or PPO active. This initial implementation doesn't actually support PPOs yet; that work is done in a follow-on CL. Bug: angleproject:3570 Test: CQ Change-Id: I534261738a55b2f671ca152c2ef9a1e81517a99c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2135931 Commit-Queue: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>