Scrolling allowed when overflow:hidden (seen on Acid2)

Autoscroll, as well as other user-driven scroll actions,
has to respect the scrollability styled into the web page.
More specifically, if a html or body tags are styled with
overflow:hidden, autoscroll should not scroll the containing document.

In order to fix this, patch hardens RenderBox::canAutoscroll as
following: previously, ::canAutoscroll was relying only on
::canBeScrolledAndHasScrollableArea to determine the scrollability
of #document node, which was unconditionally returned as 'true'.
Patch extends ::canAutoscroll to handle the #document case for
main and inner frames, and now it asks through ::isScrollable if
the corresponding document's FrameView is actually user-scrollable.

Note, that the patch changes ::canAutoscroll to cover the non-mainFrame
case now.

Autoscroll'ing the mainframe's document is hard with the current
EventSender machinary. Because of that, patch adds an iframe's document test case.
Test added in fast/events/autoscroll-in-overflow-hidden-html.html

Corresponding WebKit commit: <http://trac.webkit.org/changeset/154722>

BUG=chromium:1701

Review URL: https://chromiumcodereview.appspot.com/23450017

git-svn-id: svn://svn.chromium.org/blink/trunk@157371 bbb929c8-8fbe-4397-9dbb-9b2b20218538