FlameChart: REGRESSION zoom doesn't work properly when time window has its initial values.

scenario:
1) record less than one second timeline
2) zoom in or zoom out

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/187303003

git-svn-id: svn://svn.chromium.org/blink/trunk@168434 bbb929c8-8fbe-4397-9dbb-9b2b20218538