div with overflow:hidden scrolls when dragging in a text box

Consider the case of the following HTML:
<div style="overflow:hidden; width: 100px; height: 100px" >
  <input id="input" type="text" size=10 value="any text here!"/>
  <button style="position:relative; top: 100px; left: 100px"/>
</div>

If ones starts a text selection by dragging the mouse from within the input
field, and continues to drag beyong the outer div boundary, the latter will
be scrolled no matter its overflow:hidden style.
That happens because when the autoscroll has started, it gets propagated up
to the current layer's parent layer, instead of the to current layer's enclosing
scrollable layer.

Patch fixes the issue by hardening the way scrolling is
propagated upwards when autoscroll is being performed.

RenderLayer::enclosingScrollableLayer method also got rewritten
in terms of RenderLayer tree traversing, instead of RenderObject tree.
The rewrite adds support for cross frame upwards traversal.

Test: fast/events/autoscroll-upwards-propagation.html

Corresponding WebKit commit: <https://trac.webkit.org/changeset/154382>

BUG=116655

Review URL: https://chromiumcodereview.appspot.com/23472005

git-svn-id: svn://svn.chromium.org/blink/trunk@157437 bbb929c8-8fbe-4397-9dbb-9b2b20218538