Allow the shell runner to run outside of Mojo checkout.
Allow the shell runner to run outside of Mojo checkout.

This patch allows to experiment with the shell runner outside of a
mojo checkout. Currently this works only for Android, and only if the
shell is already installed on the device.

In the next step we can integrate download_shell_binary to pull down the
shell binary from the cloud if needed.

Fixes domokit/devtools#26.

R=tonyg@chromium.org

Review URL: https://codereview.chromium.org/1248533002 .

afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo