Add GpuCommandBuffer support.

Currently this is not actually hooked into the system.

To give some context, in a follow up CL I'll add this to GrDrawTarget.
For this I will move the gpu onDraw command to the GpuCommandBuffer as well.
For GL this will end up just being a pass through to a non virtual draw(...)
on GrGLGpu, and for vulkan it will mostly do what it currently does but
adding commands to the secondary command buffer instead.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2038583002

Review-Url: https://codereview.chromium.org/2038583002

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