Revert 232428 "Remove redundant/misleaing README.chromium files."

> Remove redundant/misleaing README.chromium files.
> 
> These files are a legacy from when the native_client_sdk
> and ppapi were DEPS'd in from an external repos rather
> than being part of chromium.
> 
> R=binji@chromium.org, brettw@chromium.org
> 
> Review URL: https://codereview.chromium.org/54983002

TBR=sbc@chromium.org

Review URL: https://codereview.chromium.org/55983003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232434 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232434