DevTools: don't allow to zoom flame chart in more than 10us window

Zoom in will stop on flame chart once the width of the visible time range is less than 10us. This helps us avoid long overlapping lavels on the overview scale and fixes broken zoom controls after zooming in too much.

BUG=426764

Review URL: https://codereview.chromium.org/683243007

git-svn-id: svn://svn.chromium.org/blink/trunk@184740 bbb929c8-8fbe-4397-9dbb-9b2b20218538