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.
Committed: https://code.google.com/p/skia/source/detail?r=4205

[Leon, 2012-06-07 15:32:59]

https://codereview.appspot.com/6297053/diff/1002/gm/gmmain.cpp
File gm/gmmain.cpp (right):

https://codereview.appspot.com/6297053/diff/1002/gm/gmmain.cpp#newcode717
gm/gmmain.cpp:717: static void usage(const char * argv0) {
This function is basically the same, but I moved it below the declaration of
gRec so I could refer to it when I add instructions for using -config.

[reed1, 2012-06-07 16:16:15]

might put an example usage in the CL comment.

lgtm