DevTools: fix rendering of markers on flame chart

Under certain conditions, we used to have a cross-like image
taking the entire canvas instead of several rows of markers.

BUG=

Review URL: https://codereview.chromium.org/235913011

git-svn-id: svn://svn.chromium.org/blink/trunk@171483 bbb929c8-8fbe-4397-9dbb-9b2b20218538