Regularize ASSERT(imp) in bindings

Currently ASSERT(imp) is duplicated in the CG
(due to matching Perl code),
and located far from the actual initialization.
This puts the ASSERT in the right place,
and removes the duplication.

TBR=haraken
BUG=345503

Review URL: https://codereview.chromium.org/201583002

git-svn-id: svn://svn.chromium.org/blink/trunk@169329 bbb929c8-8fbe-4397-9dbb-9b2b20218538