Improvements to build_command_buffer.py

Remove subdir thing for Linux. It's incorrect.

Make the script able to take extra GN args.

Default to a custom gn output dir for the command buffer build.

Default to a release build of the command buffer.

Document that the script overwrites the gn args on each run.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2472343003

Review-Url: https://codereview.chromium.org/2472343003
3 files changed