Add a dry-run option to gyp_skia.

Add a dry-run option to gyp_skia.

There are times when I want to do some low-level fiddling with the gyp
command. It's nice to start with whatever the 'default' is, and it's also
nice to peek behind the curtain to see what's going on. To this end,
a '--dry-run' in the argument list will spit out the command to run,
rather than running the command.

R=epoger@google.com

Committed: https://code.google.com/p/skia/source/detail?r=14674

[epoger, 2014-05-09 14:50:16]

lgtm

[bungeman-skia, 2014-05-09 15:04:23]

Committed patchset #2 manually as r14674 (presubmit successful).