DevTools: Use V8RecursionScope::recursionLevel instead of a Microtask checkpoint flag to find out if JS stack is unwound.

This is the way to find out if the JS stack has been completely unwound.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/122243002

git-svn-id: svn://svn.chromium.org/blink/trunk@164874 bbb929c8-8fbe-4397-9dbb-9b2b20218538