Introduce "navigation target classification" for spatial navigation

Introduce "navigation target classification" for spatial navigation

Move the functionality of Element::supportsSpatialNavigationFocus to
FocusController, by removing the call from Element::supportsFocus and
instead explicitly check for the presence of event handlers in
FocusController::findFocusCandidateInContainer.
Add a 'navigation class' to the FocusCandidate, to allow factoring this
into the navigation heuristics.

[c.shu, 2015-01-07 23:49:47]

c.shu@samsung.com changed reviewers:
+ c.shu@samsung.com

[c.shu, 2015-01-07 23:49:48]

Overall I support this code refactory. I think it even helps the performance in
the fact that many useless calls to supportsSpatialNavigationFocus() are
avoided.
With the classified result, we can further decide how to act on the candidate
elements.
I just have a couple of nits.

https://codereview.chromium.org/810493003/diff/1/Source/core/page/FocusContro...
File Source/core/page/FocusController.cpp (right):

https://codereview.chromium.org/810493003/diff/1/Source/core/page/FocusContro...
Source/core/page/FocusController.cpp:821: if (element.isSVGElement() &&
hasFocusEventHandler(element))
I would either expand the four focus event listeners here or use a helper
function for the above click/key events as well, just to make it consistent.

https://codereview.chromium.org/810493003/diff/1/Source/core/page/FocusContro...
Source/core/page/FocusController.cpp:851: FocusCandidate candidate(element,
type, navigationClass);
Can we do some additional code refactory here so that there's only one place to
call classifyNavigationCandidate()? I hope it's possible.