WebRTC: Refactor builder configs to reduce duplication.

masters/master.chromium.webrtc/master.cfg

Index: masters/master.chromium.webrtc/master.cfg
diff --git a/masters/master.chromium.webrtc/master.cfg b/masters/master.chromium.webrtc/master.cfg
index 34dd2f0250faa8440d382a8b9ce1b1358c26132b..9fff509f10ad903d76eb24640f22c015008218fb 100644
--- a/masters/master.chromium.webrtc/master.cfg
+++ b/masters/master.chromium.webrtc/master.cfg
@@ -19,14 +19,10 @@ c['status'] = []
 config.DatabaseSetup(c, require_dbconfig=ActiveMaster.is_production_host)
 
 import master_source_cfg
-import master_win_cfg
-import master_mac_cfg
-import master_linux_cfg
+import master_builders_cfg
 
 master_source_cfg.Update(config, c)
-master_win_cfg.Update(c)
-master_mac_cfg.Update(c)
-master_linux_cfg.Update(c)
+master_builders_cfg.Update(c)
 
 # Must come before AutoSetupMaster().
 c['buildbotURL'] = ActiveMaster.buildbot_url