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

git-svn-id: e8541e15acce502a64c929015570ad1648e548cd@4205 2bbb7eff-a529-9590-31e7-b0007b416f81

mrobinson/skia,mrobinson/skia,metajack/skia,Cue/skia,mrobinson/skia,metajack/skia,mrobinson/skia,metajack/skia,Cue/skia,Cue/skia,metajack/skia,Cue/skia,mrobinson/skia