Refactor base/safe_numerics.h

* Move into base/numerics subdirectory.
* Rename files for clarity.
* Add owners.
* Rename checked_numeric_cast to checked_cast.
* Fixup callsites and include paths.

BUG=332611
R=brettw@chromium.org, jam@chromium.org

Review URL: https://codereview.chromium.org/141113003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245418 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245418