firmware_DevScreenTimeout: Relax error margin

Current margin (3 seconds) is too small and thus if the device
initialize too fast, the test will fail incorrectly. Let's relax the
margin to 5 seconds.

BUG=chrome-os-partner:13530
TEST=none

Change-Id: If931902a90e730667d4b959f4a9c2518c7b49a5a
Signed-off-by: Vic Yang <[email protected]>
Reviewed-on: https://gerrit.chromium.org/gerrit/32689
Reviewed-by: Tom Wai-Hong Tam <[email protected]>
diff --git a/server/site_tests/firmware_DevScreenTimeout/firmware_DevScreenTimeout.py b/server/site_tests/firmware_DevScreenTimeout/firmware_DevScreenTimeout.py
index 57d694b..e696800 100644
--- a/server/site_tests/firmware_DevScreenTimeout/firmware_DevScreenTimeout.py
+++ b/server/site_tests/firmware_DevScreenTimeout/firmware_DevScreenTimeout.py
@@ -28,8 +28,10 @@
     CTRL_D_REPEAT_COUNT = 10
     CTRL_D_REPEAT_DELAY = 0.5
 
-    # We accept 3s timeout margin.
-    TIMEOUT_MARGIN = 3
+    # We accept 5s timeout margin as we need 5s to ensure client is offline.
+    # If the margin is too small and firmware initialization is too fast,
+    # the test will fail incorrectly.
+    TIMEOUT_MARGIN = 5
 
     fw_time_record = {}