Add documentation for using testing using the command buffer

site/dev/chrome/commandbuffer.md

+Chromium Command Buffer
+==========================
+
+It is possible to run Skia's correctness tool, dm, and benchmarking tool,
+nanobench, on top of the GL ES interface provided by Chromium's command
+buffer.
+
+The Skia tools are always built with this support. They dynamically load
+the command buffer as a shared library and thus no GYP/GN flags are
+required.
+
+The command buffer standalone shared library is built in a Chromium checkout
+by building the 'command_buffer_gles2' target. The command buffer should be
+built with the is_component_build in GN set to false. This will produce a .so,
+.dylib, or .dll depending on the target OS. This should be copied alongside
+the dm or nanobench executable built from a Skia repository.
+
+Both tools have a 'commandbuffer' config which can be used with the --config
+option to the tool and will run the tests or benchmarks using the command buffer
+library. Unit tests in dm always run on all appropriate and available backends
+regardless of the --config flag.
+