Statically assert correct types in transitional adopt functions.

R=haraken@chromium.org, vegorov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/163173002

git-svn-id: svn://svn.chromium.org/blink/trunk@167083 bbb929c8-8fbe-4397-9dbb-9b2b20218538