Reuse existing event when listening for new heap snapshots

Drive-by: removed unused _addProfileHeader

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/114143003

git-svn-id: svn://svn.chromium.org/blink/trunk@164231 bbb929c8-8fbe-4397-9dbb-9b2b20218538