Apply smack label to Crosswalk Renderer Process

I'd like to discuss here how to aplly samck label for RP.  Below is the explanation of my patch:

(1)Add an command line option to propagate the App id to RP.
(2) Insert code to apply smack lable in "content::RendererMainPlatformDelegate::EnableSandbox", before the "Seccomp BPF" really being enabled. (refer to attached patch).
The  "content::RendererMainPlatformDelegate::EnableSandbox" is invoked in the "RedererMain", so the inserted code will be executed in RP.

So far there is no opportunity to override "content::RendererMainPlatformDelegate::EnableSandbox", so we have to patch the "content" repo.

![0001-insert-code-to-ally-smack-lable patch](https://cloud.githubusercontent.com/assets/3304318/5122503/69e312b8-70dc-11e4-8291-202ad99d6225.png)

@rakuco @kenchris @pozdnyakov @tiwanek @xuzhang, I'd like to hear your comments. Feel free to override these patches if you have a good idea.