add enable_webrtc in gyp to allow some platform to disable webrtc. enable_webrtc is set to 1 by default.

BUG=none
TEST=trybots
Review URL: http://codereview.chromium.org/7450003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@93108 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed