Search MarkDOMContent in divider events only

The event is filtered out in the main record list in tracing based timeline so the test should search for it inside marker events list.

BUG=398785
R=caseq@chromium.org

Review URL: https://codereview.chromium.org/480073002

git-svn-id: svn://svn.chromium.org/blink/trunk@180525 bbb929c8-8fbe-4397-9dbb-9b2b20218538