Add sandbox support (issue #524).
- The sandbox is now enabled by default on all platforms. Use the CefSettings.no_sandbox option or the "no-sandbox" command-line flag to disable sandbox support.
- Windows: See cef_sandbox_win.h for requirements to enable sandbox support.
- Windows: If Visual Studio isn't installed in the standard location set the CEF_VCVARS environment variable before running make_distrib.py or automate.py (see msvs_env.bat).
- Linux: For binary distributions a new chrome-sandbox executable with SUID permissions must be placed next to the CEF executable. See https://code.google.com/p/chromium/wiki/LinuxSUIDSandboxDevelopment for details on setting up the development environment when building CEF from source code.

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk/cef3@1518 5089003a-bbd8-11dd-ad1f-f1f9622dbc98