[autoroll] Fix child branch name on libunwind-chromium roller

LLVM uses "main" instead of "master" for its main branch as of a month
or so ago.

Bug: chromium:1177288
Change-Id: I2427752b671124ffa8feb133d8f3abd59d32fd92
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/375066
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Eric Boren <borenet@google.com>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Eric Boren <borenet@google.com>
diff --git a/autoroll/config/libunwind-chromium.cfg b/autoroll/config/libunwind-chromium.cfg
index a9f9e3c..f8c9d96 100644
--- a/autoroll/config/libunwind-chromium.cfg
+++ b/autoroll/config/libunwind-chromium.cfg
@@ -53,7 +53,7 @@
   }
   gitiles_child:  {
     gitiles:  {
-      branch:  "master"
+      branch:  "main"
       repo_url:  "https://chromium.googlesource.com/external/github.com/llvm/llvm-project/libunwind.git"
     }
   }