Surfaces have a few methods:
1. getCanvas
2. draw(canvas)
3. makeImageSnapshot()

Clearly you could make a SkNullCanvas yourself. If you also need a null-surface, is it ok for that surface to return null from makeImageSnapshot?

Yes - it is fine to return null from makeImageSnapshot

I'll take a crack at adding this

The following revision refers to this bug:
  https://skia.googlesource.com/skia/+/44d04bd7ed3be8c39cc7ea02e3789cccf083bb55

commit 44d04bd7ed3be8c39cc7ea02e3789cccf083bb55
Author: Mike Reed <reed@google.com>
Date: Thu Jun 29 12:14:53 2017

add null-surface

Bug: crbug.com/737726 
Change-Id: Iec9094d8d7232943e90fe2d9745fc83bcdf90954
Reviewed-on: https://skia-review.googlesource.com/21190
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Mike Reed <reed@google.com>

[modify] https://crrev.com/44d04bd7ed3be8c39cc7ea02e3789cccf083bb55/src/image/SkSurface.cpp
[modify] https://crrev.com/44d04bd7ed3be8c39cc7ea02e3789cccf083bb55/include/core/SkSurface.h
[modify] https://crrev.com/44d04bd7ed3be8c39cc7ea02e3789cccf083bb55/tests/SurfaceTest.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f0e03cff8b74271d63e44f3154a86408fc5f42f9

commit f0e03cff8b74271d63e44f3154a86408fc5f42f9
Author: Alok Priyadarshi <alokp@chromium.org>
Date: Mon Jul 31 20:41:13 2017

[chromecast] Use null SkSurface instead of bitmap-backed one.

On audio-only devices there is no need to perform rasterization for
real. We just need a dummy SkSurface that simply ignores all draw calls.
Using the null surface saves 3.6M memory and draw calls are much faster.

Bug:  737726 
Change-Id: Ic11ee7afc80dd71a6ae5d5d2ba5e4aae513fcb61
Reviewed-on: https://chromium-review.googlesource.com/594643
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Alok Priyadarshi <alokp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490794}
[modify] https://crrev.com/f0e03cff8b74271d63e44f3154a86408fc5f42f9/ui/ozone/platform/cast/surface_factory_cast.cc