Remove expensive system includes from TypeTraits.h and simplify.

I checked with @thestig that it's ok to try requiring GCC 4.6, which is satisfied by all currently supported Ubuntu releases. This simplifies things.

Also, we might as well use the generic routine, even for newest MSVC builds, in order to be simple and consistent across OS'es, and avoid the expensive system includes on all OS'es.

Review URL: https://chromiumcodereview.appspot.com/23457002

git-svn-id: svn://svn.chromium.org/blink/trunk@156799 bbb929c8-8fbe-4397-9dbb-9b2b20218538