Rename currentWorldContext to currentWorldContextOrMainWorldContext

Because that's what it does

BUG=none
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/143393002

git-svn-id: svn://svn.chromium.org/blink/trunk@165411 bbb929c8-8fbe-4397-9dbb-9b2b20218538