When a program that's being used by a PPO is successfully re-linked, the
PPO's ProgramExecutable needs to be updated. This takes advantage of the
subject/observer pattern that's already established between programs and
PPOs to ensure only the PPOs that the program is a part of are updated.

Bug: b/182409935
Test: ProgramPipelineTest31.ModifyAndRelinkShader
Change-Id: Idcc11e7e819b4a9bef02bdd71afc8ea58111acc6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2770684
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>