linux: enable seccomp sandbox by default

Seeing how the bots feel about this again.

BUG=36133

Review URL: http://codereview.chromium.org/3539015

git-svn-id: http://src.chromium.org/svn/trunk/src/build@64267 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -283,9 +283,9 @@
     # better.
     'linux_keep_shadow_stacks%': 0,
 
-    # Set to 1 to turn on seccomp sandbox by default.
+    # Turn on seccomp sandbox by default.
-    # (Note: this is ignored for official builds.)
+    # (Note: this is disabled for official builds.)
-    'linux_use_seccomp_sandbox%': 0,
+    'linux_use_seccomp_sandbox%': 1,
 
     # Set to 1 to link against libgnome-keyring instead of using dlopen().
     'linux_link_gnome_keyring%': 0,