This merge request port Chromium to SlapOS in headless mode.

Some notes:

• The repo of Chromium and its dependencies are extremely big. ~19G after completely fetch. So be patient when fetch and building it...
• Use the fetch and gclient command provided by depot_tools to fetch and synchronize source code. Do not use plain git unless you understand the chromium repo deeply. Because to the dependencies relationship of chromium is complicated. Plain git may only fetch or update part of repo[1]. Then the revisions of different dependencies are inconsistent. In this situation, you have to re-do everything from scratch. Which is time-consuming.
• fetch command only could work in an empty dir. Use gclient to update code.
• Each version of chromedriver(required by selenium) only support 3 major version of Chrome[2]. It often 2 version behind the latest Chromium. For example, the latest chromedriver 2.28 only support 55-57. And the latest Chromium is 59[3]. If you want to update the version. Please make sure the chromedriver could fit them.
• The Chromium can run manually by ./chrome --headless --no-sandbox --disable-gpu. (Make sure you already use xvfb to start a display and set proper system path).
• The [chromium-download] will fetch Chromium source code in ${buildout:parts-directory}/srcdirectory. This directory created by fetch command. Then the [chromium] section will enter this repo to fetch additional libs by gclient runhooks. So the repo will contaminant by this command. Be careful this when you need to rebuild Chromium or re-run the [chromium-download] section.

1. https://www.chromium.org/developers/how-tos/get-the-code/working-with-release-branches

2. https://sites.google.com/a/chromium.org/chromedriver/downloads

3. https://blink.lc/chromium/refs/