DevTools: Track async stacks on the corresponding execution context.

We should track async stacks separately for each execution context,
so that async event IDs (like timer ID) from different execution
contexts (e.g. iframes) should not mess with each other.

BUG=272416
R=yurys, pfeldman

Review URL: https://codereview.chromium.org/111653002

git-svn-id: svn://svn.chromium.org/blink/trunk@163553 bbb929c8-8fbe-4397-9dbb-9b2b20218538