Clean-up hit-testing of scrollbars

Today there are three different ways we hit-test for scrollbars:
1) for frame scrollbars, explicitly call FrameView::scrollbarAtPoint
2) for subframe scrollbars, use HitTestRequest::AllowFrameScrollbars
3) for DIV scrollbars, rely just on the normal HitTestResult

There's apparently no good reason not to be consistent here.  This patch
updates hit-testing to always consider all 3 types of scrollbars, and
removes the AllowFrameScrollbars mode entirely.

This also fixes a bug where we weren't properly hit-testing for
subframe scrollbars on tap, and so not sending mouse events through
to the frame behind (as we do for touch).

I also split ScrollView::scrollbarAtPoint into two variants for window
and view co-ordinates (since we weren't being consistent previously,
passing view co-ordinates as if they were window co-ordinates in
RenderPart::nodeAtPoint.

BUG=410661

Review URL: https://codereview.chromium.org/527263002

git-svn-id: svn://svn.chromium.org/blink/trunk@181935 bbb929c8-8fbe-4397-9dbb-9b2b20218538