GN: Update documentation for updating the gn binaries (the rolling process).

tools/gn/docs/update_binaries.md

 # How to update the [GN binaries](gn.md) that Chromium uses.
 
-## Prerequisites
-
-You'll need a checkout of Chromium, and commit access to the
-`buildtools/` repo. Check with scottmg, thakis, brettw, dpranke, or a
-member of chrome-infra to get access to that repo.
-
-## Instructions
-
-_Hopefully there will be a script that does this all for you shortly._
-
-  1. cd to your Chromium checkout.
-  2. Create a whitespace change in DEPS and upload it to create a "dummy" CL
-  3. Run the following commands:
-    1. git-cl try -b linux\_chromium\_gn\_upload\_x86
-       -b linux\_chromium\_gn\_upload\_x64
-       -r $GIT\_REVISION\_YOU\_WANT\_TO\_BUILD
-    2. git-cl try -b mac\_chromium\_gn\_upload
-       -r $GIT\_REVISION\_YOU\_WANT\_TO\_BUILD
-    3. git-cl try -b win8\_chromium\_gn\_upload
-       -r $GIT\_REVISION\_YOU\_WANT\_TO\_BUILD
-  4. Wait for the try jobs to finish.
-  5. If they all ran successfully, copy the digests from the tryjob build
-     log output into `src/buildtools/{mac,linux32,linux64}gn.sha1` and
-     `src/buildtools/win/gn.exe.sha1` as appropriate.
-  6. Upload a buildtools CL with the updated digests and get it reviewed
-     and committed. Make sure you note the revision of GN that you built
-     against in the commit message.
-  7. Go back to your "dummy" CL w/ the change to the DEPS file in src/
-     and update the buildtools revision to your newly-committed
-     buildtools change.
-  8. Get that reviewed and landed in Chromium. Make sure you note the
-     revision of GN that you built against in the commit message, along
-     with the revision of buildtools that you're rolling to.
+Any committer should be able to do a roll by running //tools/gn/bin/roll_gn.py
+on linux or mac.