Roll gn from bb8f66fdb599 to 859dde4a7f34

https://gn.googlesource.com/gn.git/+log/bb8f66fdb599..859dde4a7f34

2022-04-02 adetaylor@chromium.org rust-project: Represent source roots properly

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/gn-chromium-autoroll
Please CC dpranke@chromium.org,thakis@chromium.org on the revert to ensure that a human
is aware of the problem.

To file a bug in Chromium: https://bugs.chromium.org/p/chromium/issues/entry

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

Tbr: dpranke@chromium.org,thakis@chromium.org
Change-Id: I9df46176a94482eeab98a60d2bbc817fdc68bd8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3566983
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Bot-Commit: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#988270}

DEPS

@@ -431,7 +431,7 @@ vars = {
   'libcxx_revision':       '79a2e924d96e2fc1e4b937c42efd08898fa472d7',
 
   # GN CIPD package version.
-  'gn_version': 'git_revision:bb8f66fdb5990ae5c0eb05152d51aca9a263b10d',
+  'gn_version': 'git_revision:859dde4a7f34a4383179522f8e1061dcffac8691',
 }
 
 # Only these hosts are allowed for dependencies in this DEPS file.