Make it possible to override the tickmarks for a WebFrame.

This makes it possible to override the default tickmark behavior for a frame.
The default behavior is to display tickmarks for find results. However the PDF
plugin handles finding itself but still wants to set tickmarks. This CL makes
it possible to specify an array of tickmarks to display.

BUG=303491

Review URL: https://codereview.chromium.org/198683002

git-svn-id: svn://svn.chromium.org/blink/trunk@169423 bbb929c8-8fbe-4397-9dbb-9b2b20218538