Add the ability to specify config to GM.
Add the ability to specify config to GM.

Code adapted from bench.

To specify a config, add -config <configName1>|<configName2> etc
to the command line.

To find the valid names (which can depend on flags passed in), use
-h or --help.
Review URL: https://codereview.appspot.com/6297053

csulmone/skia,csulmone/skia,csulmone/skia,csulmone/skia