DevTools: show aggregated stats for async trace events in Timeline

When async trace event is selected its details section is populated with aggregated stats for corresponding range. Self time of async event is always 0.

Each async event with phase "S" (i.e. begin) now has endTime as well. End time is equal to the timestamp of corresponding "F" (i.e. end) event.

BUG=405502

Review URL: https://codereview.chromium.org/619613006

git-svn-id: svn://svn.chromium.org/blink/trunk@183052 bbb929c8-8fbe-4397-9dbb-9b2b20218538