Implement ScopedFD in terms of ScopedGeneric.

Move to a new file base/files/scoped_file.h. I will also add ScopedFILE to here (currently in file_util.h) later.

I think there is a crash in the old code in content/browser/zygote_host/zygote_host_impl_linux.cc that this patch should fix. The old ScopedFD took the address of something in a vector that is being modified.

I removed SafeScopedFD from content/common/sandbox_linux/sandbox_linux.cc since base's ScopedFD not CHECKs on close failure (this is a more recent addition).

Reland of https://codereview.chromium.org/191673003/
R=agl, viettrungluu

Review URL: https://codereview.chromium.org/202113004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257473 0039d316-1c4b-4281-b951-d872f2087c98
Move CommandLine to base namespace.

Fix all forward-declares and header files referencing CommandLine.

This keeps a "using base::CommandLine" in the command line header file so that the rest of the source files can be changes in a follow-up.

TBR=sky

Review URL: https://codereview.chromium.org/196413016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257514 0039d316-1c4b-4281-b951-d872f2087c98