Fix touch scrolling of main frame scrollbars

Touch adjustment isn't aware of scrollbars, so we bail out of touch
adjustment for gesture events that occur directly on top of a scrollbar
in bestClickableNodeForHitTestResult.  In r177164 I broke this for the
main frame scrollbar case by moving the main-frame scrollbar check
from before touch adjustment to after it.

This fixes the issue by moving the check back to just after the hit
test.

BUG=403744

Review URL: https://codereview.chromium.org/492933002

git-svn-id: svn://svn.chromium.org/blink/trunk@180745 bbb929c8-8fbe-4397-9dbb-9b2b20218538