DevTools: Async call stacks for window.postMessage()

This introduces a more general instrumentation approach for tracing async calls
that could also be an entry point for public Debug API.

This patch also allows showing debugger async call stacks across the execution
contexts.

BUG=393175,272416
R=pfeldman, yurys

Review URL: https://codereview.chromium.org/404953003

git-svn-id: svn://svn.chromium.org/blink/trunk@178698 bbb929c8-8fbe-4397-9dbb-9b2b20218538