Fix main frame scrollbar hit testing when the page is scrolled

In r181935 I cleaned up hit testing of scrollbars.  The code I added
for main frame scrollbars used the incorrect co-ordinate system.

BUG=414137

Review URL: https://codereview.chromium.org/579733004

git-svn-id: svn://svn.chromium.org/blink/trunk@182176 bbb929c8-8fbe-4397-9dbb-9b2b20218538