cronet import: add --git-origin-checkout-hook to run gclient

Running gclient in Copybara's git origin checkout is a bit tricky.
Copybara does a bare checkout which gclient does not support (and it
keeps the git dir speparate from the checkout dir). This checkout script
essentially reinitializes the git dir and runs a gclient sync on it.

Test: ./import_cronet.sh -n 108.0.5359.128 -l \
1cd27afdb8e5d057070c0961e04c490d2aca1aa0 -f

Change-Id: I2b78a7b8e0e1e9adc74175e90560a58a82b978ca
2 files changed