Send a 0 idle notification on context disposal

This simplifies the logic on the V8 side that tries to guess whether the
idle notification comes from the compositor or from context disposal

BUG=none
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/666213004

git-svn-id: svn://svn.chromium.org/blink/trunk@184253 bbb929c8-8fbe-4397-9dbb-9b2b20218538