Re-land CL with WebGL fixes:
This required some extra pointer checking during validation to handle
the fact that a Program and/or ProgramExecutable may not be present when
attempting to draw. This isn't an error, just undefined behavior, which
we (eventually) treat as a no-op.

According to the OpenGL ES 3.1 spec:

7.3. PROGRAM OBJECTS
If there is no active program for the vertex or fragment shader
stages, the results of vertex and fragment shader execution will
respectively be undefined. However, this is not an error.

To handle this, if no VS or FS is present in the active Program/PPO,
we will no-op the draw command.

Bug: angleproject:3570
Test: KHR-GLES31.core.sepshaderobjs.StateInteraction
Change-Id: I70d688bf344a78cf3b4fd66c995ae03ce4b9b807
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2185156Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>