[API] Change chromium_args sperator from whitespace to semicolon.

the following case:
`"chromium-args": "--js-flags=--harmony-proxies --js-flags=--harmony_collections"`, 
only `--js-flags=--harmony_collections` work, the first one will be overrode. Change to other separator, like ; 

The new usage of this api will become
`"chromium-args": "--js-flags=--harmony-proxies --harmony_collections;--disable-accelerated-video"`