linux: enable seccomp sandbox by default

It seems the best way to keep the seccomp sandbox working is to have
everyone's development environment have it on by default.  So we turn
on the seccomp sandbox, but only for non-official builds.

If the build-time flag is set: --disable-seccomp-sandbox turns it off.
If the build-time flag is *not* set: --enable-seccomp-sandbox turns it on.

BUG=36133

Review URL: http://codereview.chromium.org/1558003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@43122 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed