[autotest] Update stable image version to R34-5278.0.0

BUG=chromium:338971
TEST=None

Change-Id: I514d60f55c7802fefd03cc2be962b4e9864e4330
Reviewed-on: https://chromium-review.googlesource.com/184356
Reviewed-by: John Dung Hong <johndhong@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
diff --git a/global_config.ini b/global_config.ini
index c1b6e1c..9be7345 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -148,7 +148,7 @@
 serve_packages_from_autoserv: True
 
 [CROS]
-stable_cros_version: R33-5116.3.0
+stable_cros_version: R34-5278.0.0
 stable_build_pattern: %s-release/%s
 # The zone that all Chrome OS devices are in if they are in a lab.
 dns_zone: cros.corp.google.com