JS binding: remove redundant type check in attr setter for TypeChecking interface

A [TypeChecking=interface] attribute has been checked already in its setter. 
If it is not nullable, just call toImpl instead of toImplWithTypeCheck to avoid 
redundant type check.

This change can improve performance and save power.

BUG=

Review URL: https://codereview.chromium.org/654473002

git-svn-id: svn://svn.chromium.org/blink/trunk@183717 bbb929c8-8fbe-4397-9dbb-9b2b20218538