This removes GetSelfPID(), GetClientFileDescriptor() and
TakeClientFileDescriptor(), along with corresponding methods on
content::ChildProcessHost, content::SandboxedProcessLauncherDelegate,
NaClIPCAdapter and ppapi::proxy::ProxyChannel. This also removes the
unused IPC descriptors kPrimaryIPCChannel and kStatsTableSharedMemFd.

BUG=659448

Review-Url: https://codereview.chromium.org/2476883002
Cr-Commit-Position: refs/heads/master@{#431036}