DevTools: instant events should not overlap on Timeline flame chart when zooming in

Instant events are now considered as having duration 1us rather than 10us which makes overlapping much less likely.

BUG=398783

Review URL: https://codereview.chromium.org/721703002

git-svn-id: svn://svn.chromium.org/blink/trunk@185217 bbb929c8-8fbe-4397-9dbb-9b2b20218538