Move V8Utilities::getExecutionContext to V8Binding.h

The goal is to remove V8Utilities.
This CL also renames getExecutionContext to currentExecutionContext.

BUG=339037

Review URL: https://codereview.chromium.org/137823006

git-svn-id: svn://svn.chromium.org/blink/trunk@166022 bbb929c8-8fbe-4397-9dbb-9b2b20218538