Check WTF::VectorFiller template argument type size in compile time

The template argument's type size in WTF::VectorFiller 'memset' specialization should be checked during compilation rather than in runtime.

Review URL: https://chromiumcodereview.appspot.com/23604022

git-svn-id: svn://svn.chromium.org/blink/trunk@157137 bbb929c8-8fbe-4397-9dbb-9b2b20218538