Account for scroll when positioning the flakiness dashboard popup

This patch updates the popup on the flakiness dashboard to be under
the cursor even if the page is scrolled.

TEST=manual, verified the popup works when scrolled in both Chrome and Firefox.

Review URL: https://codereview.chromium.org/356783003

git-svn-id: svn://svn.chromium.org/blink/trunk@177173 bbb929c8-8fbe-4397-9dbb-9b2b20218538