Remove unnecessary static_cast<ExecutionContext*>().

Document is an ExecutionContext. We don't need to cast Document objects to
ExecutionContext.

BUG=

Review URL: https://codereview.chromium.org/111323002

git-svn-id: svn://svn.chromium.org/blink/trunk@163503 bbb929c8-8fbe-4397-9dbb-9b2b20218538