Fix roll_gn.py script.

Looks like the name of the builder that builds gn has been renamed from
win8_chromium_gn_upload to win_chromium_gn_upload, so update the script
accordingly.

https://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_gn_upload

Bug: 
Change-Id: Ic3f064c554760570d5e4c3fa8796650c92440f1a
Reviewed-on: https://chromium-review.googlesource.com/752184
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#514159}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c498275a08411a20b28dca2b5434265d76eb903e