Add a GN auto-roller script.

Add a GN auto-roller script.

This CL adds a script in //tools/gn/bin/roll_gn.py that, given no
arguments, will post three CLs and three tryjobs in order to:
  * build a new GN binary at tip-of-tree
  * update the buildtools repo to the newly built binaries
  * and then update DEPS to point to the leatest buildtools

The script must be run on Linux by someone with commit access to both
src/ and buildtools/.

One can also run the steps independently for testing purposes.

It can only be run in a clean chromium checkout; it should error out
in most cases if something bad happens, but the error checking isn't
yet foolproof.

R=brettw@chromium.org, scottmg@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/1230293003

Cr-Commit-Position: refs/heads/master@{#339191}