Add a --list-features-flag command line argument to dump the features flags.

- Add a DumpFeaturesFlags to output this in the terminal.

- Fix on the way the "enable-experimental-features" string and declare
it properly in the XWalk switches.
Style fix of the vector containing the features flags.

... So that it matches the Chromium style.
Use xwalk_switches for "enable-xwalk-experimental-features"

to follow the rest of the command line arguments.