Update luci timeouts in extreme gcstress cases.

Change-Id: I4384a7d821891d202e13fd27dc27e3b43e932be8
diff --git a/tools/luci/config/cr-buildbucket.cfg b/tools/luci/config/cr-buildbucket.cfg
index 8c0125a..4fa2042 100644
--- a/tools/luci/config/cr-buildbucket.cfg
+++ b/tools/luci/config/cr-buildbucket.cfg
@@ -33,7 +33,7 @@
     builder_defaults {
       dimensions: "pool:luci.art.ci"
       service_account: "art-ci-builder@chops-service-accounts.iam.gserviceaccount.com"
-      execution_timeout_secs: 108000  # 30h
+      execution_timeout_secs: 172800  # 48h
       swarming_tags: "vpython:native-python-wrapper"
       build_numbers: YES
       # Some builders require specific hardware, so we make the assignment in bots.cfg