Set chromium.clang builder timeouts to 6h (from 3h)

These builders take a long time to finish.

Bug: 868213
Change-Id: I532603b76a6a724bbdf36f5d27e93223d6f5d2de
Reviewed-on: https://chromium-review.googlesource.com/1217223Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589982}

infra/config/global/cr-buildbucket.cfg

@@ -201,6 +201,9 @@ builder_mixins {
 builder_mixins {
   name: "clang-ci"
   dimensions: "cores:"  # Mac uses 24 cores, Linux/Win uses 32 cores.
+  # Because these run ToT Clang, goma is not used.
+  # Naturally the runtime will be ~4h on average.
+  execution_timeout_secs: 21600 # 6h.
   recipe {
     properties: "mastername:chromium.clang"
   }