This type is not supposed to be constructable by users. Internally, we
use CallSiteUtils::Construct to create CallSite objects; and we simply
map a thrower builtin as the public CallSite constructor.

R=yangguo@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2201823002
Cr-Commit-Position: refs/heads/master@{#38234}