cronet: Create bare version of import script

This creates a very basic (and untested!) version of an import script.
Checking this in now, so we can get better history as it evolves.

import_cronet.sh can be used to invoke copybara and set the appropriate
command line arguments.

Test: ./import_cronet.sh -l 108.0.5359.128 -n 110.0.5481.65
Change-Id: I74c6aa9245e102a1f94d0630e41ddd1079e9d41f