Linux Sandbox: split the GPU policies to their own files.

We split the GPU policies to their own independant files in order to make
sandbox_seccomp_bpf_linux.cc more sane.

Moreover, we clean up the logic so that the "WarmUp" process is an
implementation of SandboxBPFBasePolicy::PreSandboxHook().

More work remains to clean-up the GPU policies, but this is an important
first step.

In addition, we put all Linux sandboxing related files in content/common in
a new sandbox_linux directory.

BUG=325535
R=jorgelo@chromium.org, mseaborn@chromium.org, piman@chromium.org, rsesek@chromium.org

Review URL: https://codereview.chromium.org/99133015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240509 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240509