Convert GN hyphens to underscores when passing command line args.

Some GYP variables are named like "pkg-config"

Replace - with _ when doing converting GN variable names.

TBR=michaeln

Review URL: https://codereview.chromium.org/103193003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238494 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238494