Make sandbox code compile as "chromium_code".

This sets up useful flags like -Wall -Werror etc.

Also squash a compiler warning:

sandbox/linux/suid/process_util_linux.c: In function ‘AdjustOOMScore’:
sandbox/linux/suid/process_util_linux.c:25: error: format ‘%lu’ expects type ‘long unsigned int’, but argument 4 has type ‘pid_t’

BUG=none
TEST=try-servers

Review URL: http://codereview.chromium.org/733001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@41161 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/sandbox/sandbox.gyp b/sandbox/sandbox.gyp
index 87f4f33..50094f993 100644
--- a/sandbox/sandbox.gyp
+++ b/sandbox/sandbox.gyp
@@ -3,6 +3,9 @@
 # found in the LICENSE file.
 
 {
+  'variables': {
+    'chromium_code': 1,
+  },
   'target_defaults': {
     'variables': {
       'sandbox_windows_target': 0,