Exit with status 0 from the cp.py copy file script on success (flutter#678)

This utility script to copy a file was always exiting with status 1.

Author: whesse

build/cp.py

@@ -17,7 +17,8 @@
 
 def Main(src, dst):
   # Use copy instead of copyfile to ensure the executable bit is copied.
-  return shutil.copy(src, os.path.normpath(dst))
+  shutil.copy(src, os.path.normpath(dst))
+  return 0
 
 
 if __name__ == '__main__':