Eliminate a v8::Local in ScriptWrappable::assertWrapperSanity.

(This likely contributed to a performance regression. See bug for details.)

R=jochen@chromium.org
BUG=367132

Review URL: https://codereview.chromium.org/265663003

git-svn-id: svn://svn.chromium.org/blink/trunk@173191 bbb929c8-8fbe-4397-9dbb-9b2b20218538